Screw size/type problems- How do I know?

My problem (one of them anyway) is that each of the previous fine owners of this vehicle seem to have put a new screw in when doing work. My dash, gauge bezel, lower dash pads, each have a collection of screws I removed and none of them seem to match.
How do I know what screws I SHOULD be using and where can I get them.
I have the AIM and notice that screws have a part number. Can this part number be looked up somewhere?
I have bought some screw sets from forum vendors, but the dash (upper and lower and each of the gauge bezel screws have not been purchased.
Please help...
Most vendors offer this but no others have a forum member discount that I'm aware of.
I'll be buying them soon for my 68





