I was there a few weeks ago and can recommend the Virgin Megastore at Downtown Disney for a huge selection of everything as well as Camelot Records in the Florida Mall off International Drive in Orlando. You may want to prepare a short list of cd's/artists to take with you to save time, otherwise you might need to spend quite some time in Virgin. If you have your children and wife(?) with you, they could use the great facilites upstairs to watch individual dvd/video selections to kill some time! There is also a cd store at Pointe Orlando near Sea World but I can't remember the name of it!
Have a fantastic Holiday - We did!
Regards Geof