I had to buy one of these today-fm transmitter It got 24 good reviews and was only $7.85. It's rechargable through a usb port.
I'm thinking of coupling it with this deal it's a $4.50 charger (use promo code PAYDEALS = $4.49-----tnx sd)
If everything works good, I saved myself $50 on a fm transmitter from a name brand. (I only paid $18 bucks for my Sansa mp3 player, so it didn't make sense to pay more for the transmitter.)