Do you mean you have an aux input? If that's the case, you can just buy a regular mp3 player and get a cable (you can get it at your local Radio Shack or similar store) - one end connects to the aux input on the stereo and the other end connects to the headphone jack on the player. You won't need an fm transmitter.
If you're looking for an mp3 player, what are you looking for in a player?
Hard drive or flash memory? How much music do you have and do you want to carry it all with you? If you need something with more than 8GB capacity you may want to go with a hard drive player. Flash players currently top out at 8GB but some players allow you to add capacity by adding a microsd card or something like that.
Another consideration that may steer you in one direction or another is physical size. Do you want something small and light or are you willing to sacrifice that to have more capacity? What do you plan on doing with it when you're not listening to it in your car? Do you want to work out with it? Go running? That may steer you toward a flash player? Do you want an fm tuner that will allow you to listen to the radio? If so, that rules out iPods (unless you want to add the optional $49 radio remote).
Do you want to have a player with a lot of accessories available. iPods have the biggest marketshare, so you'll find plenty of accessories. Some other popular brands are Creative, Sandisk, iRiver, Cowan, and Archos but you'll find less accessories for these.
Oh, I guess another important consideration is how much are you willing to spend.
Give us an idea of what you're looking for so we can help you out.