If your just looking to get it running again, pick one up from a recycler or look on craigslist.
If your looking to get more performance, pick up a set of votec heads. They will bolt right on, but you will need to get a new intake to match the vortec heads and your EGR will no longer work. I'm told they add ~50 hp by themselves. I can't attest to that, but I can tell you I picked up a whole lot of power between them, a new cam, and bigger injectors.
Here's a link with all types of performace tips for that truck / engine.