Convenience stores that sell gas usually buy it from wherever cheaper. 7/11, Wawa, Quickcheck. Not sure if same applies to anywhere outside US, though.

Also local small brands may not sell top tier gas

I'll admit that the list is longer than I expected! I didn't see 7/11 on there, and they definitely sell gas. I also don't trust those little run down privately owned gas stations. I'm pretty surprised to see Arco on the list. There's gotta be a difference between Arco and Chevron. I feel like I can feel a difference in my car when I use one vs the other, but I guess that could be a placebo effect. I usually just get Costco gas for the awesome savings, but if I can't for whatever reason, I try to stick to Chevron or Shell. I suppose I can broaden my list a little.

You also want to avoid stations that take a long time to pump. That means they're not changing their filters at the required intervals, and old filters can fail. Plus, if they're not even caring for their filters, who knows what their tanks look like?