The problem with all the solutions I've seen so far is that they rely on low-resolution sources. Usually only one source at that.

Amazon has a spotty database of album art. Some new albums have super high res versions which Amazon doesn't normally let you download (easy to get around) but mostly Amazon's offerings max out at 500x500.

The iTunes store has a huge about of album art, but generally you have to go through iTunes to get it. Easy to do and on a Mac it's super easy to embed or save the album art out to files. Dimensions vary, but generally at least 600x600.

Sites like AllCDCovers have a lot of high resolution covers, but there aren't any programs out that siphon from there as far as I know. The site(s) also generally implement some form of anti-leaching mechanism - captcha, IP-based time-outs, etc.

RateYourMusic.com also has a lot of album art and it varies greatly in dimensions. Sometimes it's the only place you'll find something online though. Then there's also Discogs.net

Lastly, if you have a scanner and some time, you can scan your own. But time is definitely the big factor here because you'll also have to descreen the image to make it look half-way decent.

I use iTunes, RateYourMusic, Discogs, AllCDCOvers and of course my scanner for when I can't easily just download the image with high enough quality (or at all).