Can I change my iPhone 3GS housing?

Hi, I have recently purchased a factory unlocked iPhone 3GS, I love the phone and it is just what I need, the housing however is a little tatty so I was wondering wether I could buy a new one, and if I could fit it myself, also a rough cost would be good, I don't really trust buying from over seas so uk prices if available

Also the phone black and would like to get a white one is that possible?

White outcasing for that phone costs as little as 3 quid on eBay.