May I suggest you do something? Why don't you create your own themes yourself? PS or any other application like that can help you a lot with this task and besides, it's fun. PS can help you create themes as well as buttons.
:)
You can buy DVD backgrounds, transitions and buttons but they are kind of expensive, think they cost more than DVDA! The transition elements might be worth the price as they're rather well done, buttons as said you can easily do yourself in PS.
I've done a few of my own, just using the still camera to shoot backgrounds and put objects such as leaves into the scanner to make buttons. Now if I've got the camera with me and see some interesting texture I grab a snap of it to potentialy use as a DVD menu background.
Bob.