You might try the
Baen Free Library. They offer their stuff in a "Palm/Mobipocket" format. I haven't used the Mobi, so I cannot comment on how well done the conversions are, but the RTF's I've used have all been of excellent quality.
The stuff in the Library is, as the name proclaims, free, and if you like what you read, they offer more for sale (sans DRM) at very reasonable prices.
