Rural King is a nationwide (or almost) Farm and Home store in rural areas. They sell a lot of Cloths, tools, feeds/animal/farm products and a bit of everything.
In store, they always have pants nad shorts in the bigger sizes... Not so much on their website, at least not right now at end of the season.
I'd take a look at
DXL Big and Tall.
I'm sure they have something for you. At a quick Glance, I saw some shorts in 54-58s..