I don't think there are many dealers that would carry a lot of those kinds of parts this side of Scoggin' or the like. I believe there is an actual list of GMPP authorized dealerships listed on GMPP's website. Maybe have a look and give them a shout.
05 Hummer H2 SUV on OEM Air Ride
Forged 11.4:1 408 LQ4 (K1 crank & rods; Mahle pistons)/CPP LS3 CNC heads/BTR Stage IV LS3 cam/home ported L92 intake manifold/ported 90mm TB/custom Volant CAI/mid-length headers/AFE dual 3" CB/Corvette Servo/el cheapo lift/Cognito UCAs/Sphon poly adjustable TA/Sphon adjustable rear LCAs/e'fan conversion/DirtyMax radiator/Moroso catchcan/HPTuners/HID conversion/LED exterior lighting/LED interior accents/Autometer Cobalts/AEM WBPioneer AVH-5600BHS/Pioneer GM-9601/2-Pioneer TS-W310D4s/Obcon Labyrinth dual-12 box/lots of other little stuff