I can only think of the two 7 elevens. one in paleto bay and the other in blaine county. Why can't I go into the ones in the city or why can't I rob the check cashing place girls? Why can't I go inside the malls?
Napalms and nukes may break my bones, but machine guns will never hurt me.
You can also rob every other enterable store. Stand outside and aim at the person at the register. Then shoot the registers until they open. To rob Ammunation you have to shoot the door open then aim at the guy. He has a gun so watch out.
There's a liquor store or two you can rob. You can probably find a map of the store locations if search. Here, I will save you the trouble because I'm such a nice guy. According to this guide, there are 19 total