I used StangSuspension.com. An underdrive alone [not a kit of pulleys] was $71.95 and they charged $30.00 dollars for a belt, but when it came in it was simply a "Gates" stock number K060960, so you can skip their price and go get your own belt probably cheaper. Easy install with a strap wrench and loc-tite.
'95, white convertable, auto trans
, under drive pulley, cai, 3:73 gear. Just enough snap for a 62 year old kid! (my wife's a blond so the top MUST be down, or she won't get in!!)