If anybody knows of any other options, please add!
HBG produces a nice range of high-quality pieces in 1:72 scale. In some cases, you can even download house rules to use along with them. They are my first stop for custom pieces.
FMG produces a few unique sculpts, including a terrific set of high-quality units for Italy in the 1:72 scale. Their major products can be purchased via HBG, although most are currently out-of-stock. I’ve had trouble with the FMG website and have been unable to purchase anything direct.
TableTactics offers a good variety of expansion sets in 1:72 scale, although most aren’t up to the quality standards of either HBG or FMG. Some of their more interesting sets (the “Central Powers” variant for A&A) are out-of-stock. I’m also not a fan of the markers they use to denote infantry (stands topped by a particular weapon), but they do seem to have some unique sculpts for armor, and I have purchased the Italian and German sets. I think that TableTactics pieces are best if you’re looking for late-war and post-war additions to your games of Risk or A&A, in which case they offer some great stuff, albeit in a limited range of colors.
Shapeways has some great 3d-printed commanders, capital markers, and naval items for sale at good prices. You’ll have to paint them if you want any kind of color differentiation.
Other Games with Pieces that can be Repurposed
War! includes a large quantity of terrific, in-scale pieces from the 1800’s that can double as WW1 era, colonial, and neutral troops. The game also includes a number of moulded infrastructure counters, including ports, railroads, schools, and fortifications. The units come in sprues, and it is quite a process to get them out. Colors won’t be consistent with A&A.
Attack! has in-scale infantry, tanks, and aircraft that may suit for neutrals. The tanks are a good approximation of an inter-war model. A hard-to-find expansion adds naval units. Again, the units come in sprues. Obviously, colors won’t be consistent with A&A.
H�T makes some unpainted WW1 and WW2 infantry and cavalry in 1:72 scale. Partial assembly is required for certain pieces, but most can be clipped off the spruce and painted up, or else put directly into play. Colors won’t be consistent with A&A. Usually brown or grey.
Xeno Games produced low-quality A&A expansions. Some of their house rules are now incorporated into A&A. Their games usually give you many sets of sprues that provide a small helping each of the unit types that shipped with the original 1982 version of A&A. You also get one submarine pen and one factory per color. The colors vary wildly from muted wines to bright, tropical hues depending on which of the many Xeno Games sets you get but are inconsistent with those of the original game.
Tides of Iron I think that the infantry, which come in squads, can be repurposed. The vehicles are much too big. Some useful cardboard markets. Probably not worth the cost.
Memoir 44 offers infantry in about the right size, along with various cardboard chits that may be of interest. The artillery may also be to your standards, but the vehicles are probably just slightly too large for A&A scale.