If you are in the US, try ACE Hardware. I work on a variety of equipment and sometimes need obscure screws and connectors. ACE normally caries most of what I need. I haven't taken my N900 apart, so I cannot guarantee they will carry these.

Edit : I looked at the screws used on the N900, and noticed that two of them are torx. I don't know if ACE would have torx, but I know they have allen screws. It also would somewhat depend on which ACE Hardware you go to.