This page has a few links to web sellers and also some other good info. http://ourworld.compuserve.com/homepages/bmoura/
This site carries a big catalog of SACD's, as well as CD's, vinyl, and some equipment.
As to why there are not more titles available on SACD, I think that is a sore spot for all of us who have embraced this format, but the major labels have been painfully slow to re-release their back catalogs on SACD, not to mention the "format war" with DVD-A. But this is topic all to itself.....