Well, technically, they aren't screws--you said they were the "push-in" rivet type of fastener.
Depending on the size, you can find most of them at auto parts stores if you find they get damage in the process of removing the gauge. Once the gauge is removed, you can probably apply a fix yourself with light grade silicone--don't go crazy, in case you have to replace a light bulb in it in the future and need to be able to get back in to the housing.
This is all at your own risk, of course, as you are the one trying to solve the fogging problem--if you fear tackling it and breaking something, you have two options.
1. Live with the fog.
2. Take it in to a mechanic and pay money for the repair.
Just my 2 cents.