Does anyone know of any website that would have things such as a piano accompaniment for solo music, such as Mozart clarinet concerto or anything like that ??
Thirs, you may be able to get Google to do the talkin'. To get the above answers, I simply Googled, using quotes, "Mozart" "Clarinet Concerto" "accompaniment" "midi". I could probably find an MP3 track, if I searched hard enough. I wouldn't necessarily say you could find EVERYTHING you wanted a track for -- especially for free -- but, if it was fairly popular, I don't see a reason why not.
navigate to Mozart but you'll have to scroll down. All three movements to the clarinet concerto are there, just down the page. Revoice the midi tracks to your own taste and enjoy.
If you have trouble, speak up, I'll get the files hosted for you.