ebay for sure. I have my silver back on order myself. Paid like $19 shipped. I'm still gonna cover it with a black airjacket unless I ever get a newer macbook then I'll go clear airjacket so it'll match.
I myself bought mine used and its out of warranty so I don't care. I did the same with my 2g and made it black. It was harder to take apart iphones then, and backplates didn't have the parts on it, you had to take it off the old one 🙁