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 VMP TVS l JLT123 l O/R X l 28x11.50Rx15 M/T Street R's l BMR Suspension l Recaros
03 Sonic Blue 3.8 / 17 Magnetic Fusion Sport 2.7 EB