Unfortunately I am also unaware of a place that sells downloadable FLAC content legally and in quantity. There are a few assorted bands/albums available, but I have not come across any sort of catalog style offering more than one artist or multiple labels. My gut (and personal experience) says that the music industry would never agree to this due to the potential risks that they perceive would be associated with the format. If you attempt to find a place and succeed, I would be very cautious about the legality (e.g. if the site is located in Russia, it's not going to be legal regardless of what they say).
As for DRM, DRM can be developed for pretty much any format as it's simply a container wrapper. That being said, FLAC (Free Lossless Audio Codec) is an open and royalty free format, so I don't believe they would allow DRM as part of the licensing agreement. This is purely conjecture on my part though.
Also, as an addendum to the first paragraph,
this list might help you find some legitimate sites.