As for how to do it, you just have to have the experience. I mean, it's not too terribly hard to figure out by just messing around, I learned to do it by completely taking apart my old computer, and putting it back together. It's pretty stright forward, just pay attention to how you take your components out of the old case. As for where you can get a new case, I know quite a few places.Colorcases
has some nice cases, I got one from there a while back.Directron
has cases, but they don't have the lowest prices.Xoxide
has the coolest looking cases. They specialize in modded cases that have windows, etc. in them.
and last but not least, Newegg
. They have cases, but I've never looked at them. I just love newegg though, they have the lowest prices and their shipping is awesome. I don't think I've ever gotten something from them that hasn't come a day or more before the scheduled delivery time.
Hope that helps.