I popped into Silverprint last week and picked up a box of pearl with no problems (pearl finish seems the rarer one to find as well; most online people only seem to list gloss.)

The only problem is that most online retailers I've seen make it as hard as possible to find - the key word to look for isn't "postcard" it's "portfolio." If you see 10x15cm RC Portfolio paper, then that's the postcard paper AFAIK.

(Yes, the fact that I bought some last week means I now have no excuse not to get on with my postcard exchange prints .)