Are there any items that you can only buy from Fester (can't buy them from Joel or Gregor or find them in a chest anywhere)?

It would take some searching through the code, but we don't think Fester has anything unique. You should be able to find everything he sells elsewhere or buy it from another vendor.

I thought the pants the go with the Attacket are not available from anywhere else?

I know the Attacket is available from Joe, and is also found in a chest if I'm not mistaken. Of course I could be mistaken, and the pants are found in the chest (in which case the Attacket isn't, which would mean he really doesn't sell anything unique at all)...