I think your best bet would be an extUSB to 3.5 adapter like
this, with a standard mini-jack (3.5mm) FM transmitter. I think I've seen extUSB Y-adapters that power the device and provide a 3.5mm audio jack, but I'm not sure. I usually just stream to my car stereo via A2DP. Maybe somebody else can provide better links/advice.
Edit: Wow, I spent too long writing that. It was going to be the first response
