There have been a few Saleen Superchargers for sale in the recent past on this forum...
Also search for it on ebay or sometimes a vendor will run a "Sale"...
You should continue to look at the "Mustang Parts for Sale" section...
What will your budget allow you in terms of $$$?
Remember that the installation can be expensive as well as any upgrades you will need like: smaller pulleys, BAP, HT0 spark plugs, and a tune...
Did you have a tuner like a SuperCal II or Diablo???
I have a 3.6" pulley that I am not using now because I installed a 3.4" pulley but first get the supercharger ans then search for mods for it..
Good Luck, Dave