sucks, maybe you could find a cheap set of X5 wheels on craigslist to get you rollin.

i dunno if 18 is the smallest you can go but here are a few i found doing a quick craigslist search
BMW X5 RIMS+ TIRES
2006 BMW X5 18in Wheels & Michelin Tires~
2009 BMW X6 20" Wheels & Tires

i like #3!