As simple as this screw looks, it was tough to find for me as well. Closest thing I came across in my local hardware market was a button head stove bolt.
These handle screws originally were used together with a countersunk outside star washer. The length is critical, if too long they will interfere with the operation of the internal control rods.
I played hell getting the couple that I needed off a seriously rusted jack. The head takes a really big screwdriver which I discovered that I didn't own one that was actually big enough to fit correctly. I finally ended up using the tip of a huge straight prybar along with a wrench on the prybar flats to remove these bad boys without screwing them up to much.