As far as I know, the RSX calipers were only produced for 8-sp RSX groups, in a satin-finish dual pivot.
Every 7-sp RSX group I've come across (and I buy 'em if I can afford it) had Exage calipers.

Other than eBay, Velospace, Road Bike Review, and other sales/trade sites, I can't imagine anywhere else.

Maybe try Site Mash and enter RSX. It may come up with CL listings.