does any one know if there is a site to download more puzzles for the computer game?
I'm not really sure what you mean here. If you have purchased the computer game from this website, Sudoku The Program i.e. software by Pappocom, then you don't need to download any more puzzles as the program will come up with a new puzzle every time you ask it to. It also makes Very Hard puzzles which may suit you.
If you're talking about more books, then try Amazon.com (I assume you are in America).
Hope that answers your question.