I use the 800Mhz antenna from Radio Shack. Lots better than factory for UCAN stuff, still OK for everything else. There are probably better ones out there, but for $20 and the convenience of buying local, I have no regrets.
I also have the little tiny 450Mhz 'racing' antenna from RS. Not quite as good for UCAN but still pretty good, and the scanner fits completely inside my pocket.
If your wanting UCAN, then I agree with the Rado Shack 800Mhz antenna. For other freqs/bands, I prefer the Condor antenna. It has good "all around" ears. At times, I will use band specific antennas when concentrating on a band.