I suspect that there are a number of sites that you might try, although note that the same files may appear on more than one site.
Sometimes, a piece may have been started by one sequencer, then changed or added to by another and re-uploaded, maybe to a different site. The changes may be little more than the alterations needed to optimise the piece for a specific midi instrument, or maybe to convert a GM piece to, for example, XG.
Using Google to search, for example, for Chopin Midi file, may produce a number of results.
Another specific site which has some good sequences is http://www.Kunstderfuge.com, but this asks for a subscription although some files can be downloaded per day for free.