I found mine on autotrader. I would say its your best place to find one because you can specifically search for a manual and diesel. I was lucky because it was almost exactly what I was looking for and it was only about 50 miles away.
2004 Chevy 2500HD Reg Cab 4x4 LLY/ZF6
5" Magnaflow straight pipe, EGR Blocker, Predator on 85HP, 33x12.50's on 20x10 USA Forged Sabre's
Brotherhood of Handshakers #105
Reg Cab Club Member #44