I bought 2 sensors on ebay for 35 bucks, plugged them in and they work just fine. Took me 15 minutes to do both.
Dealer is right about the facelift sensors on the corners being a a pain, you have to drop the bumper most likely, and be aware they are the corner shaped sensor on the facelift (front corners only).
You need this, if you're handy you can get it done in an hour including dropping the bumper.
New PDC Parking Sensor BMW E39 E53 E85 E86 R50 R52 R53 525i 530i 540i M5 x5 Z4 | eBay