Don't be any kind of hurry with your device. Stop using it if possible. First of all follow the service manual for your N900 (I am just posting the link ...) to dis assemble your N900.

Our objective is to unscrew the remaining screws (which are not broken) and then to see if you can take the device apart. If yes, then it's great. If after unscrewing the remaining screws you can take apart the device, then you can see the remaining part of the broken screws easily and can then unscrew them with your hands.

Make sure that you follow the service manual in every step, and while unscrewing the broken screws with your hands, do it carefully and slowly.

Then you can go ahead and buy exactly the same screws from where ever possible. Do not try to use screws of different length or size as replacements for the broken screws.

Also, when you finally assemble the phone and tighten all the screws, make sure that you don't over tighten the screws at all. As soon as you feel that rotating the screw driver is tightening, just stop screwing then. Going further might damage the screw holes or break the screws.