Upload & Sell: Off
This tutorial should give you an idea of the installation that you have to do. It's actually a fairly easy procedure with a steady hand and a pair of tweezers. On the models that Ian mentioned above, Canon's made the process even easier and offers the replacement screens. The ones that you see on the internet are usually modified versions of these.
If your old screen just fell out… you might want to make sure that the mounting bracket ("spring lock" in the above link) is stil around. It holds down the screen, and replacement screens usually don't ship with them.
People (me included) have had variable luck using the cheap ebay focusing screens. I have one in my 20D, which has the same mounting bracket system as yours. the focus screen is off - the camera backfocuses by a touch when the split prism is lined up, but it's still significantly better than when I tried to focus f/1.2 lenses with a stock screen There are ways to fix this, but it involves putting in shims to move the focusing screen position.
If you focus wide open and stop down before the shot, the split prism usually works very well. When I use my 17-85mm f/4-5.6, the situation's a bit different. At f/5.6, half of the prism is fairly dark. Moving your eye up and down helps this, but you always have one side of the circle darkened and it can sometimes be a pain to use.