The most common sizes for 16.5" replacement are 245/75R16 or 235/85R16 (therefore they are usually the cheapest) . The 235/85R16 is about an inch larger diameter, but it still fits fine.
For a C30 you also need to look for load range E (10 ply rating) tires, if you want to keep the load rating for your truck. The 245/75R16 size is very common on half ton trucks, but most standard tires won't have the load rating you need.
I have a K25 Camper Special, with an 8400 lbs load rating that originally had 16.5 x 9.50 tires. I usually just shop around for size and load rating E in 16", and see who has the best out-the-door price. I don't do any off-roading, so I look for a moderately aggressive all terrain tread. I am currently using Firestone Destination A/T 245/75R16E.
Bruce