Q: Where can I get non-GMO heirloom Big Boy and Better Boy tomato seeds?
A: When I saw your question initially, I thought to myself “What does he mean by ‘heirloom’? Those tomato varieties aren’t that old!” But I am going to have to eat my words! One definition of heirloom tomatoes includes those varieties introduced 50 years ago, which would include both of these!
Addressing the first part of your question, to my knowledge there are no GMO tomatoes available to homeowners, either from seed or at the grocery. Some have been produced in laboratories but they are not commercially grown. And to your second query, you can get Big Boy and Better Boy tomato seed from Totally Tomatoes (totallytomato.com). To reward you for helping me learn something new, I posted the history of Big Boy and Better Boy tomatoes at http://www.walterreeves.com/gardening-q-and-a/big-boy-vs-better-boy-tomato/.