Gonna be hard to find a regular bolt for that...those are what is known as "pipe thread", and they also have a tapered center. You might look at your local parts house for a "brake bleeding kit", that comes with plastic/nylon fittings that screw into the ports, then you can plug the ends of those with a rubber hose/screw. Some of the kits also have block-off fittings, for that purpose.
Otherwise, it's pretty much hit/miss on finding bolts that fit, without damaging the tapered ferrel inside the master cyl.
If nothing else, get a short piece of brake line from the parts house, and crimp it shut.
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