You'll be hard pressed to find spare heads in 1/25. There are dozens in 54mm and 1/35, but none show up in a search at a prominent figure site. You might try finding a site that sells Jimmy Flintstone figures or stock up on race crew kits and Campus friends from Hasegawa and Tamiya. There might be some 1/25 Germans from Tamiya out there as well from the days of their 1/25 scale tanks.