Uncle Will's Full Throttle produces 33" wheel sets Fox Valley produces 33" and 36" wheel sets I don't know of anyone who produces 38" wheel sets
These are a bit on the pricey side but they are 5.04 mm DIA (43.6 " DIA), wheels I think... http://www.searails.com/powermaxinfo/wheelsets.html
Is there a market for sizes other than 33" and 36"? I will have 45" but as a split axle version and used as a 33" Nn3 wheelset in a locomotive. That makes them 45" Z scale. RP-25 profiles and S-4.2 dimensions.