Find stores that sell produce in bulk and always keep bags and/or containers in the vehicle you use to go shopping.
If no one sells in bulk where you live, suggest the idea to the managers of the stores you frequent. It may be that they don’t know that their customers would like that option and all it could take is for customers to speak up.
But, even if stores don’t rush to open up a bulk section, you will still have done something important: You will have planted a seed in someone’s head that things can be different from the way they are. Every single little action counts, especially if many of us take it up.