The part number for the original 18" rim for K models, is 43003-30 (chromed). The rim is the same as a 45 rim, but chromed. This is much easier to get after market than the 19". The 19" though used through '56 for K, KH, KHK,'58 to '63 XLCH. These should come up on EBay once in a while.
I think V-twin still lists the Chrome 18" 43003-30.