I used to like PAW (performance automotive wholesale), but I don't even know if they're still around. Summit and Jeg's probably have them, as well as just about any auto parts supplier worth their salt. Autozone, O'Relley's, Pep Boys, NAPA, all should have basic (and not so basic) re-ring and re-seal kits available.
As to what else to do? For sure change the timing chain and gear set. Don't even bother to go all that way and not go that extra "little" bit.
How many miles on the truck? If more than, say, about 80-90k, I would actually do a full valve job too. You can do exchanges that are relatively painless these days. Well worth it in my opinion. I did mine at 98k and the engine ran worlds better than before!
And no, you shouldn't need a valve job that soon. But sometimes you do. And you'd be surprised sometimes what used to wear out on these older engines. Valve seat heat-treating was not always consistent, and what with the lead all gone out of the fuel in the last thirty plus years, the seats take a real beating.
Have fun!
Paul
