Check online tire/wheel retailers as well. If you don't HAVE to have a matching colored wheel, you can find them online. The ST175 80 D13 size tire on a five x 4.5 bolt pattern is a popular size. I got mine for my Tracker PT175 trailer online and kept the spare when moving over to my G3HP180. Here are some examples:
http://www.sturdybuiltonline.com/ST17580D13-Bias-Ply-Tire-mounted-on-a-White-Painted-5-bolt-Wheel-_p_69.html
http://www.palmettospecialtytire.com/servlet/the-191/ST175-fdsh-80D13-Loadstar-Trailer-Tire/Detail