Has to be Ipod???
I you're not talking about the KCA420 interface, but rather just a 1/8 inch jack (headphone jack) to RCA adapter, then any MP3 player could connect to that, but you have to manually change the tracks on the iPOD.
I picked up my KCA420 for $45 shipped on ebay. Here's one for a similar price:
http://cgi.ebay.com/ALPINE-KCA-420-I...temZ9729528227
You can also pick up ipods pretty cheap. You can even pick one up with a bad display, load 10, 15, 20, 30 or 40 GB of music on it and forget about it.
Check out the price of these iPODs:
http://search.ebay.com/search/search...&so=Show+Items
Edit: Also to answer your question you can get adaptors to make auxilary inputs for the headunit (rca cables) to use with any mp3 player, although you wouldnt be able to controll your mp3 player via the head unit. Also if you do it this way you will loose sound quality (shouldnt be too much) if you use the headphone jack on any mp3 player and split it to rcas.
Edit: Also to answer your question you can get adaptors to make auxilary inputs for the headunit (rca cables) to use with any mp3 player, although you wouldnt be able to controll your mp3 player via the head unit. Also if you do it this way you will loose sound quality (shouldnt be too much) if you use the headphone jack on any mp3 player and split it to rcas.

