The ST235/17R15s tires you referenced are not a standard sized trailer tire. They're a very difficult tire to find. Your best bet would be to replace them with a more common tire like the 225/75R15 tires which are extremely close in overall dimensions which I will put below.
235/75R15 tires are 9.3 inches wide with a height of 28.8 inches.
225/75R15 tires are 8.9 inches wide with a height of 28.3 inches.
The Westlake ST225/75R15 Radial Trailer Tire # LHWL304 is a load range E tire... view full answer...