I'll disagree that Manybooks titles look good (no authors, straight from ASCII, etc.) but add (for Kindle books that DO look good):

2,000 or so from the forums at Mobileread.com
25,000 from munseys.com for download via USB cable
25,000 from munseys.com/joomla for download via the web browser, and
maybe 600 from boysbooks.org (specialty site for, well, young men).

Remember to pick books formatted for "Mobipocket."