I think you would need to burn the tunes from within itunes to a CD. You can then rip the CD using any ripping software (Windows Media Player will do the job) and save the ripped files as MP3.
itunes downloads are MP4 format.
Most free MP3 downloads are of unsigned bands, so the songs are not widely known.. You will need to Google for those sites as they are too numerous to list here.
Nearly all legal download sites now charge per track as opposed to a single payment allowing any number of downloads.