I agree with Jason J on both SACD and DVD-Audio - it's pretty dependent on the actual mix. Some of my SACD's are exceptional, while others sound just like CD's.

To add to the excellent suggestions for places to find discs that have already been offered, I've gotten some of my SACD discs from Tower Records' online store, some of my DVD-A discs from Amazon.com (although they are often not good about identifying the discs as DVD-A), and at least one SACD straight from Telarc.