I have recently revisited toy soldier wargaming as a place to focus my energies - you may have seen my computer-assisted rules for toy soldier wargaming or read my toy-soldier rules Edwardian Splendour. When trying to assemble armies of 40mm/42mm or 54mm toy soldiers for wargaming purposes, you very soon come to the point where - unless you have a lot more money than most of us - casting and painting your own toy soldiers starts to make a lot of sense (and some of us simply refuse to wargame with plastic figures - it's unnatural!). As of this writing, using the more-expensive metal sold specifically for drop-casting toy soldiers, each 54mm casting will generally cost between $2.00 and $3.00 USD. This is about half the cost (or less) of buying the castings already made. (If you want to mix your own alloys, and buy lead, tin, and bismuth in their pure form, the price drops by a factor of about five.)
There are many good videos on YouTube for doing your own casting. I also include instructions (and a discussion of casting alloys) in my primer for sculpting and casting custom figures. In my experience, casting miniatures requires care, but is a relatively easy thing to do. You will, of course, require moulds, and the listing below gives all those with which I am familiar. Some of the older metal moulds are more difficult to work with - I have noted where these are the ones being sold. Most moulds today are made of hard silicone rubber, which is much easier to use. Moulds are organized by their sources/manufacturers.
Prince August not only make traditional toy soldier moulds in 54mm, they also have a wide range of more detailed 54mm moulds for the Napoleonic era and the Seven Years' War. They have an excellent line of knights in 54mm - these are great moulds, with lots of interchangeable parts for creating unique figures. In addition, they have several series of moulds for 40mm semi-round tricorne soldiers, including a full-round artillery piece. Semi-rounds use less metal, and have a very distinct "old-fashioned" look to them which is great for toy-soldier wargaming. Among their latest releases are a series of moulds for multi-piece 40mm SYW, but also the Karoliner (for Marlburian), French Regiments, and Wild Geese series are useful. They also have some old cowboys and indians moulds in 40mm semi-round, and some 40mm Vikings and Saxons. (They also have lots of small 25mm Napoleonic moulds in 1/72 scale, and a few - the Austerlitz series - in a more modern 25mm/28mm scale, plus a series of Celts and Romans in modern 25mm/28mm.)
Prince August is based in Ireland. Their moulds are available in Germany from Berlin Zinnfiguren (see below) and in the US from Dunken (see below).
There are a very large number of mould series available from Castings, almost all in 54mm scale. These notably include a number of toy soldier series featuring traditional Britains-style guards and highlanders, some more realistic highlanders (to match Prince August), Napoleonic French and British, Victorian Civilians, Cowboys and Indians, American Civil War (including a Parrot rifle, good for Edwardian conflicts), knights (many from the Britains Deetail line), and modern soldiers and Afghans. The moulds are silicone rubber. Castings also has some excellent metal moulds if you are willing to deal with them. Many of these are in smaller scales. My favorites are of a German Marching Band, which are semi-rounds in about 40mm, but include a mounted figure and a marching soldier with rifle, all in pickelhaub.
The Dunken Company is the Prince August distributor for the US, but also has a number of additional moulds which may be of interest. They have an extensive series of WWII moulds (you may recognize some of the poses from plastic army men you owned in your youth). They also have an Alamo Series and a large series for the American Civil War, neither of which are covered by Prince August.
Dutkins Collectables produces moulds in two scales - 25mm and 54mm. These are both named the "Lil Army" moulds, and the extent of coverage is impressive, especially on the 54mm side. Of note are the traditionals, colonials, ACW, WWI, WWII, and moderns moulds, but he also has knights, ancients, French Foreign Legion, Napoleonics, and on and on. Although the pictures on the web site can make the figures seem very unrealistic, this sometimes seems to be a function of the painting and photography. The moulds which I have used sometimes produce very realistic figures. They generally do have a very distinct "toy soldier" look to them when compared with Prince August, for example, however.
As of this writing, the Dutkins site is offline as the proprietor closes his physical shop and shifts to an mail-order/online business model. Check back, however - the range of moulds here is very impressive. All are done in a very durable silicone rubber.
Most of the toy soldiers in the US and the UK focus on 54mm as the standard toy soldier scale. In Germanic countries, this is less true. They invented toy soldiers, and for a long time 30mm (for flats) was the most common scale. Many producers also created 40mm/42mm full-round figures - these were much more common in Germany and Austria than elsewhere. This is reflected in the moulds which are available.
Berliner Zinnfiguren does not produce moulds, but they are the best source for many of the less-common manufacturers. I have ordered them from the US and always found their service to be prompt and reliable. They are the German distributor for the Prince August moulds, but they also have several other lines, described below.
Nürnberger Meisterzinn produces metal moulds for the SYW and Landsknechts in semi-round 40mm. They are very much toy soldiers in appearance. Many come with swappable heads to make more than one figure.
Schildkröt produces the Zinnbrigade moulds for 40mm full-round figures of the German Empire c. 1900. These are great for those looking to find 40mm figures for the late Victorian/Edwardian period. They also produce a casting of a Krupp gun, but they do not sell the mould for this. These are silicone rubber moulds.
Creartec (Artidee) are moulds in two scales of interest. They have 54mm full-round moulds for Napoleonic subjects (but be careful - some Napoleonic moulds are 90mm) and they have a useful civilian lady in a bonnet. In 40mm, they have full-round moulds for SYW and landsknechts and a series of semi-round moulds for the Napoleonic period. The moulds are silicone rubber - the ones I have are of a softer rubber, and also require a metal base like some of the old metal moulds (a piece of flat anodized aluminum sheeting works fine).
Nowikoff produces rubber moulds in full-round 28mm (Napoleonics), 45mm semi-round (Napoleonic), 54mm full-round (Napoleonic and SYW, and ceremonial German WWII). Horses and riders are sold as separate moulds.
If you are aware of other sources of moulds not listed here, please let me know and I will add them. While we are faced with a fair amount of choice, when it comes to toy soldiers there will never be enough!