The best prices on the Borla cat-back I have found were at North Coast Mustang. However, they were a bit of a hassle to deal with. Buying it from a sponsor would probably be quicker and easier, and best of all, it would support the Jet.
One note: when you buy it, it will probably arrive in the wrong package. For some reason, Borla sends out a lot of exhausts with the wrong application marked on the box. My 2004 2.3 liter hatchback system arrived in a box marked "ZX4 '05 2.0", but it was the correct exhaust.