Unfortunately the dealership is likely your only option. HOWEVER, not all dealerships are created equal (their pricing and service are excellent indicators of this). So shop around, and find a good price. eBMWparts.com (which is a dealership) was doing keys for $160 for a while.
1999 Techno Violet Metallic M3
Bilstein | Kosei K1 Racing | Toyo Proxes R888 | BMW CCA #430881