I know a place to get the Bilstein BTS for $713 shipped, I think stang suspension is around $740 without shipping. But I'd have to look for it only if you really wanted the BTS.
I got mine at stangsuspension.com, awesome store. You could always check Ebay too. Either way it will be fairly pricey, but so worth the money - never has my car felt so solid when taking hard turns, and its very stable at highway speeds. Bilsteins FTW
I paid right around $600 for my set but for some reason Howbar Motorsports doesn't come up on the internet anymore. They are a good shop out of California.