For me it was parents of my kids' friends, often via sports or other group activities that the kids were all involved in.
Those friendships fade quickly once the kids grow up, by the way, as there is no longer the common connection. I've made some attempts to maintain these, but they generally are not reciprocated and at some point you just stop with the one-sided effort.
I have not discovered the secret to long-lasting friendships, if there is one. They all seem to rely on common proximity (school, work, gym, etc.) and they don't really hold up if and when those things change. I have not had any friendships that ever lasted very long past graduations, job changes, moves, retirements, etc. I think that's basically natural, and that friendships really depend on some outside set of factors that keep people coming together for the same things.