I use a iStuff AutoScan FM Transmitter which is not too bad - I run it from the Power socket in the boor and bought a 2 metre extension canble to plug in to my mp3 player in the arm rest
The sound is a little muddy, I think these transmitters don't have enough bandwidth to give a crystal clear sound, but it isn't bad, and anyway it's in a car - not the best audio environment in the world
I got it from iworld here:
iWorld - Please enable your cookies
£24.99 - not bad at all and good enough for my use!
HTH