My JVC El-Kameleon has RCA inputs on the back, no aux-input adapter required. Some of the newer ones do require adapters, but there's lots of the older ones out there with RCA in on the back.
If you can't find one with aux-in, then look for one where the required aux-in adapter is cheap and easily available (like a Kenwood with CD-changer control - you can get the aux-in adapter from Crutchfield for $20).
Some of the decks (like Aiwa) have the aux-in on the front - while this works, it's ugly as you need to have exposed wires.
Go into a car audio shop and ask which ones have aux input. In my experience, a lot of salespeople don't really know, and they'll sell you a deck with RCA inputs on the back, but it won't work without an aux-in adapter (Sony decks are like this) so ask them to hook something up the the aux-in and shw you that it works beofre you buy.
Best of luck,