For a Kenne Bell supercharger you have to go directly to them.
KenneBell.net
http://kennebell.net/products/supercharger-kits/mustang/kenne-bell-supercharger-kit-mustang-46-26-28-1996-2004/
If you buy one, don't go cheap and exclude the inter-cooler. Those things get hot so you will need to...