I agree that this forum and feedbooks are the best places to start, but if they don't have the book you want then a site like
ManyBooks.net will automatically produce a Kindle/MobiPocket version from the original on-line source. Quality isn't consistent, because there is no human in the loop, but these may be good enough. Note that, so far as I know, Kindle and MobiPocket formats from ManyBooks really are identical. Combining the two, this is now the 2nd most popular format at ManyBooks (
2008 format stats here).