I'm actually doing the same thing right now.
Here's where I'm looking, in order...
Walmart - if they have it, it's 500 x 500
Next I look at Buy.com - clicking the thumb shows the full size image, but to download it you need to right click in the thumb, select properties, and highlight the http of the actual image. for example, this is listed as the address...
javascript:largeIM('
http://ak.buy.com/db_assets/large_images/779/60146779.jpg');hide('ppForm')
and you only want this part...
http://ak.buy.com/db_assets/large_images/779/60146779.jpgpaste it in the address window, hit enter and you'll get their 500 x 500 image
Next is Amazon, they're pretty hit and miss, usually 300 x 300 at best,
Next is discogs.com; they're great for trance/house/ambient/whatever, but they also have a lot of mainstream stuff too. Takes a bit of digging to find large art, but they have quite a bit.
yahoo image search comes next at
http://images.search.yahoo.com/search/images/advanced takes you to the advanced search, and I look for large images, then medium if there are no others
I've done some google searches for some wierd stuff, but what I have above gives me most of what I'm looking for.
I'm always looking for other good art sites, so if you know any, lets have em...