Falcon question

I found out what was wrong with the trans jumping out of gear, the screw that goes into the shifter and is underneath the shifter, also has tightining properties because the screw place has like a place where it can slide up and down but keep the shifter in place, my screw is missing and that's what causes the loose shifter and in turn, jumping out of gear sometimes, where can I get the screw? I'v got a dearbourne classics book on the way but I was wondering if it was safe to use like a regular machine screw as long as it was the corect size and length. I also tested the charging system with 1 of those 14.00 cigerette lighter digital testers from walmart, says 14.1 volts on full charge when following directions of holding the car at 2500 and check, dosen't go above nor below so it seems to work good.

Tomorrows Vetrens day and I have no school so I can post a pic of the shifter but, it's not a floor shifter, it's a colum shifter, the screw that's missing is just underneath the the shifter, inside the colum but is assessabl through a slot that it rides in from the outside, it's missing but you can tell it had a screw in it. It goes into the gears fine, the shifting itself isn't sloppy just the fact that the shifter itself wiggles up and down and not so much side to side. I'll deffenetly get 3 or 4 pics of the shifter to be on the safe side.