You are in tne usa try this place.
You do not want to go to a smaller tire, the load capacity of an 8.00x16.5 tire is 2045, whereas the 8.75x16.5 is 2680. Putting on the smaller tire will lead to an unsafe condition.
If you want to change then go to 9.50x16.5 tire which has a load capacity of 3195; the section width is 9.5 inches compared to 8.8 inches, or .7 inches wider, and a difference in overall diameter of 1.1 inches--30.6 vs. 29.5. The larger diameter will effect the speedometer slightly, but that is easy to check out to see how much slower it reads than what you are actually traveling.
Firestone carries the 8.75r16.5 load range E tire, It is listed at $156.00 retail, but I am sure that you can get it cheaper by browsing the internet.
I increased the tire size on the rear of my 1987 class A (8x19.5 to 245-70x19.5) and it made a world of difference in the handling and only a two mile-per-hour difference on the speedometer. The tires are too big to put on the front so I am stuck with the narrow 8x19.5 tires there.