Yes,viton material is excellent,for some application Buna-N is enough.No, in fact, you can't even buy the o-rings separate from Ford AFAIK. They just sell the whole valve.
Any place that sells o-rings can fix you up. Viton is the preferred material, but Buna-N has been working for me for quite a while (over 1-1/2 yrs). For Viton o-rings, try www.mcmaster.com or www.oringsusa.com