They sell some sort of device that you plug a portable MP3 into it,
and that thing sends the music to a blank radio frequency so the mp3
player can be heard on the car radio.

What are these things called?

I'd like to get one, but dont have a clue what to shop for. I asked a
guy in the electronics dept at Walmart and he could not figure out
what I wanted. (Of course I didn't expect anyone with knowledge at
Walmart), but thought I'd try. I suppose this is something to buy
online, or maybe Radio Shack...

But first I need to know what they're called....

Thanks