Also make sure you are looking at trailer tires instead of car/truck tires. The sizes look the like same sizes as older car tires, but they are different tires. Trailer tires have a lower load rating, but may cost more than car tires because the sales volume is lower.
235/85R16 is a very common tire size for 10 ply truck tires, but that would be overkill for most trailers. I'm not sure they make them in a standard C load rating.
Figuring out good replacement sizes for older tires always has a few alternatives. The original tires on my truck were 9.50x16.5, which turns out to be about 31" and suitable for the stock 16.5" rims. However, I am using 245/75R16E tires (31") and 16" wheels instead because they are very easy to find. I could also use 235/85R16E, but they are more like 31.5".