http://www.dpdproductions.com/page_vhf_air.html#airomniThey're all sort of on the expensive side, but worth looking into if you don't have the time or inclination to build your own antenna.
I have not evaluated any of these antennas. Your performance will likely be more limited by your local terrain and distance from the airport. And possibly local noise conditions, like power lines noise, etc.
Disclaimer: I have no connection with or commercial interest in DPD Productions. YMMV, etc.
Dave
