tryind to find a sevice manual

my boss asked me to try to find a sevice manual for these more common trucks we work on. I have down loaded manuals for my motor cycle, but I dont know where to look for truck manuals. any help would be great...
the truck is a 99 GMC C7500 Topkick with a 7.4 gas engine..

Blown R1

Can you not order the complete GM service manual from a dealer? They are a little expensive, but more complete than any other manual ever printed.

try www.helminc.com