The local Amish community sounds like a good start, to finding a wheelwright who can build what you want. Being able to take your vehicle right to the shop, might be handy for the builder, seeing what hubs are needed, etc. You will need to know that hub information if you shop for wheels at sources further away.
Witmer's Coach Shop in PA, is a source for wheels. Your size might be a
stock item or they could get them built for you and ship.
Witmer Coach Shop
1070 W Main St, New Holland, PA
Another person recommended these folks, they were happy with small wheels they got there.
Stutzman's Wheel Shop LTD.
Noah M. Stutzman
33656 Co. Rd. 12
Baltic, Oh. 43804
330-897-1391 ext. 1