Sites for CD-quality and studio master downloads:
Qobuz: This French site is now the world's best megastore record store for downloads. While the catalog is not quite as deep as Amazon or iTunes, it offers a vast wealth of recordings available as downloads in CD quality and often hi-def. Plus it has a wealth of unbelievable bargains, although you have to search for them.
HDtracks: Although the selection is fairly limited, this pioneering U.S. site has hi-def recordings you won't find anywhere else and is especially good at providing astonishingly rich remasterings of classic '50s and '60s jazz, rock and classical. Forget about bargains.
Pristine Classical: Historic classical (and a few jazz) recordings in public domain remastered to the highest modern standards. Wilhelm Fürtwängler's "Ring" recordings sound like they were made yesterday (or at least the day before).
eClassical: A decent collection of downloads, CD quality and higher, at decent prices.
Presto Classical: A British site with a strong collection of CD-quality downloads but not hi-def. Many European releases are blocked, but the site also sells CDs and video. Many bargains.
High Definition Tape Transfers: Reasonably priced historic recordings, classical and jazz, transferred directly from reel-to-reel tape. There are many rarities here and the sound is gorgeous, but the selection is small.
Hyperion Records: The boutique British label sells downloads of its recordings in CD quality and as hi-def studio master often less expensive than you can find on other commercial sites.