Try this thread over at spj and let me know if it helps you out.
http://www.smartphonejunkie.com/showthread.php?t=181897
As for buying music, I usually either buy the cd or have a friend make me a copy of their cd. Honestly i can't think of any other music stores other than the popular ones on the web such as iTunes, Amazon, etc. But I would prefer to have the Sprint Music Store on my phone as well since that's what it comes with stock. I hope that thread turns up something for you.
Edit: The only other way I can think of to get it is to pull it from the stock Sprint rom. That may or may not be easy. some of the stock sprint apps are in .cab files in the "windows" directory on the stock rom. It's definetely worth a try since i'm sure you will be updating your rom soon anyway. If you choose to flash back to the stock rom to look for it, just sync your phone with active sync in XP or Windows Mobile Device Center in Vista and you will be able to browse the system files on the device and copy the cab if it is in fact there. Another thing you might want to check into is if Sprint Music Store will even work in WM6.5. I do not know if it does or not sorry.