They're wood. The 1968 cube pieces are wood. If that's OK, just google "risk 1968 game pieces". It looks like several offerings (both full games and pieces only) are on eBay an Etsy right now.
Hasbro also released around 2010 a retro version of Risk with 1968 wooden cube pieces as part of their Vintage Games Series. So unless you are dead set on having the pieces from the original 1968 set - locating a copy of the Vintage Game version may also be an option (and potentially cheaper).