In our local store, they are on a shelf just down the aisle from the keys. Don't know if other stores are arranged the same. Also don't know if they have the gauge, but I'll look next time. I already keep a few gauges on the bench and in the shop.
I remember being at a small hardware store in Flagstaff AZ back in 2015. I don't recall if it was an Ace or TruValue Hardware. I told the guy I was looking for some 8-40 Fillister head screws for my TC Encore forend. To my amazement he asked what length I needed. He proceeded to show me a rack of pullout bins in the fastener section. They were labeled gun screws and there must have been 6 or 8 drawers full of oddball screws in common gun sizes. They also had an enviable selection of small springs.
I live about 3 miles from our local Ace Hardware. They have gun screws, Even have some torque and Allen head screws for bases and rings. I think it is a lot who orders the screws, If someone working in the store is a shooter, It seems they have a better selection of gun screws.