Well, the standard audio and video cables are Yellow, White and Red. The HD Cables are called component cables and they are Yellow, Blue, Red, White, and Red. When you get your HD screen, you should just ask one of the employees for "HD component cables". He should get you what you are looking for. As for price, I have seen them go from $15 to $60.
EDIT: If you are not in the US, things should still work the same, but I don't know the money conversions.