Vortec heads can be bought from GM performance parts, or any parts counter, but I think Ebay is your source. If you want new heads, I suggest calling Dal Slabaugh at 1-877-448-5451. He's at Van DeVere Pontiac Buick in Akron OH, so just ask for Dal in parts. He is the hookup for car parts on the cheap (or at least less than retail) I'm not the expert on them, but do a search for them on the net and you can get casting numbers, etc. They were widely used in trucks for the last several years of the SBC so they should be a very common salvage yard item. I suggest you do more steps of research beyond me since there is rumor of a "bad" vortec head that doesn't flow quite like the others. You can get them as either bare castings or complete assemblies. I would probably buy the bare castings and assemble them since they probably don't have the springs you want anyway. Vortec heads won't take really high lift numbers so you have two choices; limit your lift and supplement flow with duration, or have a machine shop turn down the valve guides so you can have more lift. The latter is more expensive, but not much and it prevents you from wasting time and power on less than ideal cam numbers.
I've built three engines. The first was a rebuilt pontiac 389 with a very mild cam that was one step up from stock. It was pretty awesome and ran forever. The second was a Caddy 500 that I had built with a 204/210 cam that is exceedingly mild in that much displacement. It peaked its torque right around idle (no kidding) and was completely done by 3800 rpms. It acted more like a diesel. The third is still in progress. Its a 454 with a 212/218 cam and some nice head flow. It should make 400+ hp when its done, but I keep hitting little troubles with it.