http://www.trailerandtruckparts.com sells various trailer wheels. Easier to search their site if you can find the mfg. part number.
If it's a SenDel wheel (it'll have sendel molded into the back side of the wheel) you can browse their aluminum trailer wheels at
http://sendelwheel.com/wheels/aluminium-trailer-wheels When you find the style, open the page for it and you can then find (at the bottom of pg) list of sizes, bolt patterns, and corresponding part numbers (along with offset, center bore size, load rating).
For example, mine is the T03BM. On the page for that style you find that 15"x6" w/ 6 lugs is mfg. part number T03-56655BM. Searching T03-56655BM on trailerandtruckparts.com website and I find I can order it for $105.50.
Hope that helps.