Your 15" trailer tire is most likely an ST205/75-15 or an ST225/75-15.
We offer both sizes in a BIAS & RADIAL. 15" BIAS-PLY Trailer Tires ( "/D15" ) are typically lower in price, and are very cost effective for trailers not used on a daily basis. 15" RADIAL Trailer Tires ( "/R15" ) run cooler and have a wider footprint. More tread touching the road = longer treadlife.