a six speed? unless you have an 03 cobra, mustangs didnt come with 6 speeds. you can get a T5 (dont know year, so just saying T5) tranny from a junkyard, off www.stangparts.com and some other places. just look around.
however if you really want a 6 speed, get a T56 and then spend a lot of money putting it in. youll need a new bellhousing, a chip so your car knows it has a 6 speed, a new driveshaft, and some other stuff