Hello John,
In a nutshell there is
wide variety of flat figures available from dozens of manufacturers. The figures generally range from 30mm to 100mm+ and pretty much cover all historical eras from the prehistoric to modern as well as fantasy/sci-fi. The historical figures cover all nationalities as well, but most focus on European history. There are large ranges of American Civil War pieces, Native Americans, etc. The traditional flat is engraved onto a slate block and cast in a white metal alloy. Some are two-sided. Pieces are sold as a stand alone figure or in sets. You can also get accessory sets (cannon, field equipment, etc.). Other flats are being cast in resin, but these are a distant second compared to the metal pieces. Other people (such as myself) scratch build their own. Most flats do not come with painting guides - written or illustrated. Additional research will be necessary if you are painting historical pieces - a Napoleonic era French Cuirassier for example.
Most of the vendors are in Europe and most of those seem to be in Germany where flats originated. You'll find a list of engravers and casters in the 'Links' above. I have not dealt with any of them (more on that further down). Much of the flat industry is a garage or kitchen-based enterprise. Some are larger than others and some are better at illustrating their wares better than others. Their web presence ranges from nothing to fully illustrated catalogs. Judging by comments on this site, there are a few who don't speak English, but most seem to be pretty good at getting back to you when you have questions.
In the US, the top dog flat vendor is The Little Tin Soldier
http://www.thelittletinsoldier.com/catalog/ . The Red Lancer
http://www.redlancers.com/ also has a
few flats. I use the LTS for anything I don't make myself. The service is great and shipping is both reasonable and timely.
On a side note, flats are cleaned up/trimmed using the same modeling tools you use for plastic, metal,and resin models and figures. All should be primed and can be painted with artists oils, modeling enamels (Testors, Humbrol) or acrylics (Reaper, Vallejo, Andrea).
Do you haven any specific genres or scales you're interested in? It'll help narrow down the answers; otherwise, we'd fill a book...
Whereabouts in the US are you?
Cheers ...and welcome to the crazy. New guy buys the beer.
Glen
...and what Charles said.