Here’s the ones I know:
http://www.bogartracingwheels.com/
http://www.kodiakracingwheels.com/
http://www.circleracing.com/
http://www.ccwheel.com/
http://www.fikse.com/
http://www.forgeline.com/
http://www.realwheel.net/
http://www.keizerwheels.com/
http://www.diamondracingwheels.com/
Also Tire Rack can do custom 3-piece wheels by O.Z.