Check EBay. There are several listings. Just had a twin turbo setup for about $400 bucks. I am looking for a single turbo for my 5.0 Just have to be patient. How much of the work do you want to do?
http://www.allfordmustangs.com/ is a great site for info and systems.
You have to decide exactly what performance you are looking for to determine the turbo/supercharger question. Typically you get a better ratio of HP produced to HP used to drive boost on a turbo (top end) but can get better boost off the line with a super.
Good luck with your UPGRADE!!!