As far as I know, Home Depot still sells WHITE asbestos shingles. (If you don't see them, ask for help at the Building Materials Desk.) I believe Lumber Yards can get them too. Just be sure, if you order them, you order the right size. (I don't think they come in any other color but WHITE. Usually, they're painted ...)
Anyway, if you're looking for a replacement, as far as I know, you'll have to replace the ENTIRE house! I'm not sure what mixing and matching, even look-alikes, will do when Mother Nature gets done w/them over a few years. If you have more ?'s, e-mail me directly, or Post up.