welcome to the fun of getting parts for a second gen pontiac f-body,
Try going to here
http://www.showcars-bodyparts.com/firebird70-81.html
they have lots for stuff for our cars.
I was just thinking and I don't know of any actual ground effects for these cars. Unless you are refering to the fender flares.
a few other sites to try for info
performanceyears.com
pontiaczone.com