I have a 2008 MCS and want another set of wheels and tires, but I apparently need to find some tire pressure sensors now as well.

Tire Rack sells them for $200 for a set but they are on backorder and the date they are showing as getting them in keeps changing.

My dealership sells them for $644.52 a set and they don't have any either. Not that I would buy them at that price anyway.

Fastmini.net sells them for $360 a set, but I don't know if they have them.

Any other sources to find these? Why are these things so expensive?