Some places sell the entire kits - bumpers, side scoops, spoilers, etc.
They tend to be cheaper overall, but again, check for quality. You don't want something that's going to fail or not fit right 2 weeks after you have it painted/installed.
Cruizin Concepts has the kit - bumpers and side skirts for 399.99. I don't know about their quality though, you may want to do some research.
Saleen Body Kits 99-04 Mustang Replica
12 Black GT500 SVT PP l 2.6 Pulley/Idler l Airaid l o/r X l 305 NT555R's l Recaros l
04 Azure Blue Mach 1 / 03 Sonic Blue 3.8 / 14 Sterling Grey F150 FX4 402A 5.0 / 11 GT500 R.I.P.