That is a socket head cap screw. Try Fastenal or Graingers.

@hamfast At least I have the right name for the class of products now, thanks.

Grainger wants to sell me a pack of 50 screws. Is there a place I can just buy two screws?

Probably not. You also need them in stainless steel for outside.

McMaster-Carr

12.9 is the highest strength. Metric screws also have different pitches (distance between threads). 1.5 is the pitch.

Have you tried you local hardware store? What about a big box store? Both types of store sell by the piece.

I have seen them at Lowes.

https://www.lowes.com/search?searchTerm=allen+bolt+head

While you can sometimes find a knowledgeable person in a big box store, usually there is someone in your local hardware store that can help.

Don't bother with looking at the blister packages you probably will not find them. HOWEVER, this type thing is usually found the the rows of small drawers at both Lowes and Home Depot. If the local hardware store only has blister packs you are going to the wrong hard ware store.

@krnuttle I did spend time looking through drawers at Home Depot, but you are right I need to find a knowledgeable person there.

@hamfast That boltdepot website had it. It was hard to find as their default socket cap screw selection was for US measurement but I found metric and had it ordered in five minutes. Overall an excellent website that made it economical for me to order two bolts, quickly. I bookmarked that for the future.

I hope it works for you.

Just documenting this for others who follow later: metric bolts are measured by the distance from the point UNDER THE HEAD down to the end. So remove the bolt head from the total length measured. I ended up ordering 65 mm bolts when the measurement without the head should have been 55 mm.

FWIW, all bolts are measured from under the head to the bolt end.

Did you get a Class 12.9 bolt? Class 12.9 bolts are a made from a hardened alloy steel. In contrast, regular stainless steel bolts will only be about 55% of the strength of a Class 12.9. This doesn't matter in many applications, but in critical situations it matters greatly. We had maintenance folks at work that 'fixed' a corrosion issue by replacing hardened alloy steel bolts with stainless steel and then asked why the equipment occasionally broke bolts and ended up on the floor.

@HU-867564120 Yes boltdepot had the 12.9 bolts.