"Stomach ache" is a condition, not an object. Remove the "a". "Do you have something for stomach ache?"

Shouldn't these be accepted also? do you have something for a sore stomach? do you have something for a stomach ache?

alternatives.... do you have something for a sore stomach? do you have something for a stomach ache?

"belly" is not accepted.

Given brzuch is belly, bellyache should be an option here.