You can get new, American-made wood furniture for FALs from Ironwood Designs (just Google them). Otherwise your option is to find surplus military wood furniture manufactured many moons ago in Austria, Britain, Canada, Australia, India, or Israel. Proper fit is iffy, and as you might imagine, condition is a big issue. It's very hard to find a whole set of matching pieces -- stock, pistol grip, and handguards -- in decent shape.
Legality is also an issue. There are very exact rules/regulations about the number of imported parts allowed in FALs built in the U.S., so you have to keep that in mind when replacing furniture.
Imbel is a Brazilian arsenal that manufactured FALs for both the Brazilian military and international markets, under license from FN in Belgium (the original designer/manufacturer of the gun). Their standards were always high (still are, in fact). FAL builders like 'em a lot.