You may find these sites of use.
Classical music
www.youraccompanist.com
Very good quality downloadable accompaniments to many of the most popular classical arias and songs.
www.opera-karaoke.com
A site specialising in orchestral accompaniments. Again good quality and inexpensive.
If you want sheet music an inexpensive resource for music out of copyright is
www.imslp.org
You can find nearly all the operas in the world in the Petrucchi library which is linked to this site.
Popular music
There are many resources but I use
www.ameritz.co.uk
This has many many backing tracks for popular songs.