Trailer tow mirror
 

What and where have people found mirror extensions for towing.

you need to go to a RV/ Trailer dealer for the towing mirror. They have tons of selections.

The OEM BMW mirror made for towing is made by this company. The BMW part number is 51160421840

http://www.realoem.com/bmw/diagrams/i/a/16.png

I've priced it out from the dealer, its just over $200. I was going to purchase it for towing my boat, but I realized I didn't need it.

The mirrors that you find in the RV/ trailer place are of very similar to what you are showing but nowhere near $200 and are of good quality. You are right, I do not think you need it though. I don't use my X5 to tow but I had a Jeep Grand Cherokee that I used to tow my 22 ft travel trailer and never had the need to use the tow mirrors. Now, I have a 1 ton truck to tow my trailer.

^^ The BMW ones don't vibrate like the cheaper ones you can buy from a trailer shop.