Metal Adventures, the folks who brought you BronzClay, will be bringing out a copper version this spring. (another reason to look forward to spring!)
If you just can’t wait, Hadar Jacobson has a powder product, just add water, available on her website : http://store.artinsilver.com.
Complete mixing instructions are on her site, including a video clip.
While you’re at her site, be sure to check out her books as well.