Welcome!
I'd start with museums (science, aquaria, zoos, art, history, children's), many college-associated afterschool, weekend, or summer programs, 4H, Lego robotics clubs, afterschool programs that participate in Kangaroo Math, etc. (for some of these, search the national/international organization, and then locations near you). Museums often have children's programming, sometimes associated with homeschoolers. Your local library almost certainly has book clubs. You can also try bookstores.
Your region is highly likely to have some math afterschool tutoring places (Russian Math-type places), but those may or may not be populated with enthusiastic learners, rather than parent-driven high-achievers.
Did you have a rough age group/level in mind?
