you wont find a set of oem ones for cheap. Someone just sold their complete set for $600 because he was parting out quick.
http://www.ebay.com/itm/BMW-E90-E91-...5f56bc&vxp=mtr
Thats a complete set
OR
you can buy the housing and the mirrors separate
http://www.ebay.com/itm/BMW-E90-E90-...102fb8&vxp=mtr
http://www.ebay.com/itm/BMW-M3-GLASS...ad2f0c&vxp=mtr