eBay has some good deals now n' then, but you also gotta factor in the cost of shipping ... and the risk of the occasional scammer.
Comb the local ads for cars being parted out, or start hitting up the local boneyards. You can use a T-5 out of a four-banger 'Stang just fine, and they always can be had for a THIRD of the price of a V8 T-5, just so long as you either use a conversion pilot bearing (easily obtained through eBay or other Internet sources) or change out the input shaft, and you don't make a habit of powershifting it and/or pushing some major mods on it (power adders, built motor, etc.)
You could swap in an old C-4 tranny, but there'd be more cost/labor/parts involved than it would be worth, and there'd be no blessed 4th gear for highway driving.
(Btw, Hanlon Motorsports kicks arse. Got one of their input shafts in my 'Stang. Quality stuff, nice folks, and decent pricing.)