If you mean that you have a head unit that capable of playing ipod/iphone then go to your car audio store to get it. They will know the model as it is rather head unit brand specific sort of. I'm using alpine head unit and searched high and low for it before finally found one
Which reminded me i need to get another head unit complete with that cable for another car otherwise wont buy. Go for popular brand of head unit and it would be easier

Alpine can cost you around 700-1.5 and more. Depends on whether you are real audio head or just need to listen to nice song decently.
A lot of options now actually such as sony, jvc, kenwood etc. If not mistaken the entry level that have ipod features excluding the cable is around 300-400. Cable most probably around 100-200 more. Bring your phone as well as what sets another apart is features that it can have. For example ability to read and display id3 tags, able to show the cover album etc. I will be trying to find another one in about 2 weeks time after get back in KL.
No shop shop recommendation unless you are audio head then you can go to kk lau etc. Otherwise normal accessory shop like brothers, superstar will do. Try to hunt it now since we just had cny so these shops tend to have sales. They would probably carry the cable as well or at least one of their outlet will have it.