wifi solution for 16 acre ballpark

Hi - I need someone to point me generally in the right direction.
I'm on the board of youth sports league and we would like to have wifi access. This is primarily important for our own use in order to run cash register equipment in 3 locations in the park (2 concessions and 1 fan store). I've been unable to find a standard router that we can use because of the distance involved, but I know it must be possible. The complex is 16 acres total, although in terms of distance the 2 concessions are about 750 feet apart with the fan store about 100 feet from one and 500 feet from the other - roughly speaking, the 3 buildings form a triangle.

There may be some simple answer that I am completely overlooking, but can anyone help point me to the easiest and least expensive solution? We are a non-profit so any cost savings we see is great for us. I'm attaching a picture for clarification of the area.

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