Exactly what Bryan said. You can only buy what you need, but it all helps. Buy what you can, let friends and neighbours know what a great product it is (obviously if appropriate).
Be a nice customer - talk to the staff and let them know how great they are! Feedback on the product. If there is something that would be better for you - bigger/smaller packs, different packaging, delivery, whatever - let them know. Most small businesses are operating on guesswork and don't have a marketing expert. Be a tidy customer, if there's a spill let them know discretely.
I suppose you might be able to advertise on their behalf. In terms of telling friends and neighbours, social media can be a powerful tool, so if there is a local FB page or other local online noticeboard you could share your positive experiences there. Share any media they make online: FB, instagram, whatever. It's hard to get heard on these platforms if you don't have the volume, so local ones will probably be more powerful. If there isn't a local online magazine then maybe you could be the one that starts one.
I don't need to preach to the converted, but shopping locally really does help the local economy. The big guys won't even notice you, but you make all the difference in the world to the small guys like me.
(My little store)