I know its not easy to find here in US but I got cb750 with 17inch rear but i can get 17'' 4.5 firestone coker tire only 16'' or 18''.
Ive seen some cb650 has 18'' inverted comstars. but no for sale.
Wanted 18' inverted comstar rear rim with a brake hub.
These wheels came out on honda CB750K models
like on this bike
http://www.bikez.com/pictures/large.php?image=http://www.bikez.com/pictures/honda/1981/17716_0_1_4_cb%20750%20k_Image%20credits%20-%20Kakan.jpg