got a ? on a tire size

i have a friend wanting some 33 x 13.50 x 20 tires. cant find that exact size, anyone know a company making this size or a size close to it? any help is appreciated.

http://search.ebay.com/search/search.dll?from=R40&_trksid=m37&satitle=33+x+13.50+x+20+&category0=

Thats an odd size.. Why that size? most people like the 33x12.50-20.