Does anyone have a really good online source for high-quality album covers?

I've been using Buy.com because they have crisper/larger images as opposed to Amazon.com.

I have almost 300 covers downloaded (my collection + my sister's so far) but wonder if anyone has REALLY hit pay-dirt and stumbled across any sort of database or anything . . .

Ideal size for iTunes is 4.75x4.75 @ 72DPI. The Amazon images are good for the small window, but look terrible when the larger image window opens.