I've have a better solution!
1. Find the album you're looking for on the iTunes Store in iTunes.
2. Right click the artwork and select copy link.
3. Paste the link in Safari, I'm using Plaid – Scintilli as an example here:
4. Right click the artwork on the webside and select "Copy Image Address", you will get a link similar to this:
This is the small album art, to get the large version replace 170x170 with 600x600 like this:
I've tried it for every album that iTunes couldn't find with "Get Album Artwork" and it worked flawlessly.
Please mark this as a correct answer, thanks!