Ron,
Maybe I'm just misreading the intent of the title, but I think you may be mis-stating the problem. It's not that 15" wheels are hard to get, at least not at this time. It's that a car made for 17" wheels may not be able to fit 15" wheels over the stock brakes. That's the area on wheel sizes I would focus on in putting together a list of IT(U/R/Q/Alpha) cars. You might also take into account common wheel widths in those sizes and looking into race tire availability is a good thought, although somewhat hard to predict what the future may hold.
~Matt Rowe
ITA Dodge Neon
NEDiv
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