What I do when album art is not available on iTunes or does not match up, for whatever reason:
Do an image search on Google for the album. Copy the best image I can find.
In iTunes, select all the songs in the album, right click and choose Get Info. Go to the Art tab and paste the album artwork. In iTunes 11 you can right click on the album cover in Album view and choose Get Info instead of selecting all he songs individually.
The next time you sync, the album artwork should get transferred to the iPad.
If I've got a lot of artwork to fix, I just put the browser and iTunes next to each other. It's possible to just drag the artwork from the browser to the Get Info section of iTunes. At least it is on an iMac.