You have to google, and use different search engines.. There are free ones, which range from suck to great. some with a lot of detail, some just somebody banging on a piano.. I sometimes go to Yamahasoft. Their midi files are pretty accurate, but are programmed for one of their instruments.. So you might have to reassign MSG/LSB/PROGto get right sounds.. Unless you have a Tryros, avoid those cause they use mega voices, which have notes in octaves in 7-8 which play artifacts, string squeaks, pops, click.. this will sound like spurrious high notes out of tune on a tradtional patch.. you can easily edit these high notes out, and track..
Keep seaching net for midis.. I seem to get some different sites. I friend of mine buys very well made MP3', which he sings to (but not midis)