You ask them. Seriously. You go to all the local stores in person (call first and ask to speak to the manager) - big and small. You tell them you'll do your best to get publicity. Call the local papers - especially small weekly papers. Send out press releases. (Lots of examples on line.) Tell the store owners you'll have signs at the beach when you are doing your projects listing who the donors are. In my experience you'll get more yesses than nos. Another good way to get publicity is to invite a local bigwig (mayor, council person) to come to the first day and pick up the first piece of trash.