[mr48chev]

In simple terms bleeders for pre 1975 or so American Usually had 5/16 or 3/8 Sae threads and the length and taper on the end would be the differences. If you have a thread gauge with your tap and die set check the thread pitch on the bleeder screw.

Best idea is still to take the wheel cylinder and or bleeder screw or what is left of it to a NAPA store and see if they can match it.
O'reilly's lists bleeder screws but may not stock them in most stores. Autozone also lists them but I expect the same, you have to know what you want to order it.

Dorman shows them and one thought is that if you have an independent parts house in the area with a large assortment of Dorman cabinets you might find bleeder screws there. One of my student's and his dad had a parts house and the whole front side of the conter below the counter was lined with Dorman cabinets. I scored odd ball items more than once in those cabinets but after they sold out the new outfit never restocked them and the last time I was there the pickings were slim.

I may have replaced one bleeder screw in all my years of working on brakes for a living and that was only because someone had rounded the head off using the wrong wrench.

All bets are off if you have a foreign made replacement wheel cylinder though. More likely than not they will have a metric thread on the bleeder screw.