I get mine from the local hardware store. I took in one of my screws, and showed it to the man, and he went to a bin in an aisle where they had lots of `em. It's a kind of "set screw" commonly used to tighten handles and knobs on shafts in many kinds of devices.
In designing their stuff, Leo Fender and his people incorporated a lot of stock industrial machine parts. Over the years, and with changes in where things are manufactured, and so on, the threads, and allen-wrench socket sizes have changed, but the parts are still basic machine hardware.