.flac is more common. There are a few sites that sell classical albums lossless, expensive. Beatport does electronic music and you can buy lossless there (although at an overly expensive price too, might as well buy the record if available). As far as mainstream, I don't know of a good one.
I try an stay on the straight and arrow and still buy CDs. Brutal, huh?
I know someone with a consious that buys off iTunes/Amazon and then downloads the lossless torrent from a torrent site. Hehe. Crazy in my opinion, but, mainstreams perception of quality is so careless that I think it's a non issue for anyone except our nitch.
I use foobar2000 (Windows) to play .flac files, btw.