Amiga Kit stock them. You can pretty much guarantee that there will be one on Ebay any given time too.
If you want to use WHDLoad to run games from the hard disk I would recommend getting a RAM board or 1230 turbo card with RAM.
A quick search of Ebay in the UK brings up 2 turbo cards and a 8MB board.
A good alternative to a hard disk is to use a Compact flash card and a CF-IDE adapter.
A lot of people install laptop CD drives in the A1200 too.
If you're prepared to pay a bit extra and do all the above you'll have a nice little games system.
