These wheels are pretty awful, and likely not usable for any design which requires live wheels, but try anyways:
http://www.amazon.com/Fairbanks-905-.../dp/B0026GFQZA
On further research, it seems that Banebots wheels come in a 4 7/8" size, which I'm assuming will work for your design.
http://banebots.com/c/WHB-WS-498
However, I warn that they're a pain to mount. That being said, they're fantastic wheels. Their driving surface is among my favorites.
Good Luck!