We used this one that Glider recommended for my husband's bike, it seems to work fairly well for when he forgets the ipod.
Hidden Antenna For Touring Bikes - Harley Davidson Community
If you do a forum search for 'hidden antenna' you will get about 2 pages of threads to read through. :s
I have tried 3 different types of hidden antenna. All give pretty good reception if your not to far out in the country. I now have a marine type hidden and it works as well as as any I have tried. I picked it up off ebay for about $40. I also picked up a dual antenna plug so that when I have the box off the back of the bike I can use the whip antenna, which gives good reception out in the country.