subaru, NASIOC (always at least one for sale on there), ebay but forums are gonna be where you will find the best deal. and you probly want an '04 tranny unless you wanna swap all your axles and hubs and get new wheels.
Nasioc has a lot of them. I've got a 6spd driveshaft if you're interested. $100 since you're a wrxatlanta regular. Any 6spd should be fine, they changed the axle designs somewhere between ver7 and ver8 trannies but as long as you've got the right axles it will bolt right up.