Now that I have a reason to purchase DVD-Audio discs, I've been searching for online sources. It looks like the DVD-A Registry is a place where you can see a list of everything that has been released on DVD-A. It also looks like they include a couple other formats like Blu-ray. There are some other formats I'm not familiar with like MVI and HDAD. If anyone has info on these please feel free to contact me or leave a comment.
Update: I just got some more info from Feisal, the creator/owner of the DVD-A Registry. Here's his response:
"MVI is dead & gone - it's a CD+DVD with hi-rez stereo and possibly a 5.1 DD track. There were only about 15 MVI discs released ever so its really not that important. HDAD and DAD were coined by Classic Records. DAD was a DVD-video format disc with 24/96 stereo audio only, and HDAD was a DVD-V on one side and a DVD-A on the other side (with 24/192 stereo MLP)."