DVD-Audio is a bit tougher than SACD (fewer new releases and more older releases out of print), but both can be found at Elusive Disc. Other online stores also carry both formats, but they are often difficult to identify - Amazon.com, for example, does some weird stuff when identifying both formats since they have no official categories for either SACD or DVD-A. There is also SA-CD.net as a resource for SACD reviews.