People have a right to buy alcohol on Sundays if they choose. As others have noted, businesses have the right to choose not to sell alcohol on that day if they wish. The right to buy alcohol does not require that anyone sell it to them if they feel they should not. Is that a difficult concept to grasp?
You must live somewhere else. Where I live, it's against the law to sell liquor before noon on Sunday, also after one on Friday and after midnight on Saturday, I think is the way it goes. Except for bars, which cannot serve after two a.m.
There are no rights outside the law. I suggest you not test that in TX, for your own welfare.