If you have a tire size in mind and it looks like you do, then you might try tirerack for some good tire selection. They have user reviews for just about every tire they sell.So I'm thinking of changing to 20" rims, 8,5" wide. What would be the widest tire I can use on these rims? I think 285/50/20 approaches the stock diameter the best, but there are only crappy tires available in that size ( over here in Europe that is). I prefer normal street tires, the M&S Firestones I have now are way to noisy.
Once you pick a tire that meets your liking you can click on the "specs" link, and get all the details on that tire, or maybe find a size that will fit your rims better.
I'm pretty sure they do international shipping, and I believe Steve has worked out some kind of discount for GMTruckClub.com members.