Intense-Racing.com
ZZPerformance.com
Getoverkilled.ca - currently only doing tunes, only sells parts in spring/summer
3800performance.com
theres alot, what all are you looking to get from the car? Goals? Budget? Are you doing the work yourself, if so how good are you w/ a wrench?
I can give you some ideas of where to start, etc once i know a little more.
As for interior, any GM Wbody 99+ seats should be a direct swap, what are you looking to do for the interior? LoTek sells a 3 pod A pillar.
Exterior, there isnt too much you can do body kit wise that looks decent. Theres a few different grilles etc that look OK IMO. Wheels are 5x115 w/ a 35-45 offset.
Suspension- GMPP and BMR Fabrication make good kits, KYB agx(lowered) GR2 (stock ride height) are popular strut repalcements.
Brakes - F-Body caliper swap is a popular mod (dual piston alum as opposed to your single piston cast iron)