Generally it's easy: Do you have auto-dimming (electrochromic) exterior mirrors? If so, it's going to be more expensive. If not, it'll be pretty cheap, since they're just heated. Just make sure you have the correct side (passenger) mirror.
Plenty on eBay:
BMW X5 X 5 e53 Spiegel Spiegelglas Glas ELEKTROCHROM Außenspiegel Rechts | eBay
BMW X5 X 5 e53 Spiegel Spiegelglas Glas ELEKTROCHROM Außenspiegel EC rechts | eBay