It seems like no owners or tuners are interested in an 18" alternate wheel as most are focusing on 19's and 20's. Unfortunate state of affairs. It appears that those that are planning or those that are attending track days are doing so on street rubber, not R compounds.
Given the larger front brakes, the backspace dimension and spoke shape is going to be an important factor in finding the right 18" wheel. All that is available now is previous generation wheels run with spacers.