I got mine from ebay (UK)- just searched 100percent food grade cocoa butter, its said to keep 6 months refrigerated but i'm skeptical, I think it would last for ever as its only vegetable fat really! (I'm very happy to be corrected though!)
A little cocoa butter goes a long way, melt a little in a small glass jar, mix in colour with a brush and sit the jar in warm water whilst working with it.
In adding oil- too much stops it from hardening when cool, too little and it hardens on the brush- for me 1 heaped teaspoon of grated cocoa butter to three drops oil works
For a more opaque colour add a little white powdered colour.