Does anybody know where I can buy online (non-downloadable) the Watkins
Shaw edition (vocal score) of Handel's Messiah and miniature scores of
Faure's Requiem and Beethoven's Missa solemnis?

Perhaps a better question would be to ask for good sheet music stores
online.  There was something in San Francisco, but I can't remember its
name.  Patelson's in New York has great selection, but they're not online.
I've tried some of the sites listed on ClassicalNet but have so far come up
empty.

Steve Schwartz