Replaced my 3GS screen, not sure if the process is any different from a 3G.
Can't remember where I ordered the screen pack but it one of the first couple google results.
Found the process of replacing the screen pretty straight forward, only trickly bit was removing the LCD screen from the front panel without cracking the LCD... So I guess just be extra careful and do it slowly.
There was no replacement rubber seal which had to be removed but isn't all that noticeable without it.