Here's a stupid idea: 18" diameter wheels with black lip to simulate 15"!

Why hide the rim to make a faux look? Just buy some good looking 17's and be done. They don't look too out of place and tire selection is good and affordable. 18's and up are when things start to look wrong and that's mainly because they all seem to be overly modern and very tacky.

Rims can make or break the look of a car.