Pretty well every tackle shop sells frozen squid, but some of it is pretty ropey. The unwashed stuff is best, buy it in bulk and split it into smaller packs. But if you have to buy pound boxes, try to get it from a shop selling it for human consumption. The quality is likely (but not guaranteed) to be better.
I believe that if you buy it from a tackle shop it attracts VAT as it's bait. Buy the same thing from a supermarket/fish monger and it's classed as food and is exempt from VAT. That's one of the reasons it's more expensive in tackle shops.