yeah dont worry about it , most places will just give you sizes to keep both front and rear the same , or they sell packages with cheaper sizes to keep the advertised price down but lets face it when your buying 20's or really fat 18's for the rear you know its not going to be cheap when you go down that road in the first place. its all just advertiseing to entice you to buy bigger rims , same rason volks arent that easy to come buy at your average wheel place ,i mean you put up a head line price for a 18's thats more than 1k ex tyres than the erst are in 20" wiht tyres its scares the kids off.
then add in the fact that most cars are fwd and wont take more than 8" up front then you look at some of what the yanks put on the Z and you see why they dont stock the stuff we need.