On Shopify's end we have an app called DandyLoop right from our app store, that connects various businesses and their products to be shown together. I would recommend trying that out there!
Looking a bit further online, I was able to find a few strategies and guides on cross-promotion, and found that there are a couple services like crosspromotionpartners.com (Not affiliated with Shopify) that seem to have a listing service for businesses interested in connecting.
I hope that gives you some connections you could use. :)
Enjoy your weekend!