Hmmm try
www.gmpp.com.
www.summitracing.com sells the complete long block as well.
Or search for GM performance parts.
I believe any GM dealer can get the GMPP 383.
But I think their long block is kinda lame for a 383.
It does come with a roller cam but it is pretty mild.
To me it kinda defeats the purpose of having a roller cam with such a mild grind.
Personaly the only reason to have a roller cam is to take advantage of being able to use a radical grind unable to be accomplished with flat tappet lifters.
The couple of extra ponies is not worth the money involved with a roller set up to me.
When in a regular rig set up, is not gonna see above 5500 rpms that often anyway.
But that's just me.
For the price of a roller set up you can get another cam with a better grind and be way ahead of the game.
Playing the if I had the money game..
383 rotating assy frome GMPP
Rhoads cam
Ebrock RPM manifold
WP's Sportsman II heads
Q-jet to top it off.
Good bottem end (stock converter if in front of an auto) and good mid and top end.
I think that combo would give a nice fat and flat torque curve. Which in a heavy rig is what it's all about.
Build for torque... horsepower will take care of it's self.