What a sweet idea! Here are some resources:
https://sustainableamerica.org/blog/how-to-start-a-garden-share/
You could also ask around on your neighborhood email list or Facebook or next door to get contact information for your nearest community garden. Some have wait lists, so you could potentially ask to offer this spot to those on the wait-list. If they were already interested enough to join that wait list they might be more motivated than just some random person on a whim who will let weeds take over.
Or ask direct neighbors. I want to garden more vegetables but don't have sun. Our community garden is not close enough, but I would love to tend a sunny bed next door and would happily give produce and flowers to the owner.