I moved a lot during my life, living in the 4th country now, stayed in each one for about 15 years. Therefore I had to find friends from outside of school.
Here is a list of how I found them:
20's:
through my cousin, he was a coworker with my future best friend
started a band with a friend and his cousin, then we kicked out the friend and got a better drummer. Through the band we met a ton of other musicians while playing life and became friends with them
I joined a IRC channel about writing HTML and CSS, once a year we met in real life. Over time I became closer friends with some of them and we visited each other semi regularly. We still hang in the same chatroom, over 20 years ago. Half of us moved abroad, so we can't meet IRL that often anymore, but we are still friends
after moving countries I made a house warming party and asked my (now ex) wife's brother to invite his friends, I brought 50 liters of beer from Germany to this party in Sweden. This group became my core group of friends even after the divorce
30's:
work, I became very good friend with one of my coworkers, we even started a new company together because I was the only one who wasn't afried to try it
university, yeah normal
one uni friend pulled me in to the company he worked for where there were very many super cool guys and I became friends with many of them. Even now like 7 years after we don't work together we still meet regularly for grill parties, etc.
40's:
after another move, to South Korea, this one is tough because I still don't speak the language, but after we got our son, my fiancé opened a public group on the Internet for couples who have a small child and one of the parents is a foreigner. Many of them don't quite fit me as friends but we still meet some of them for play dates and so on so our friendship is growing
I was on the playground and there was another foreigner dad and we started talking about the kids and everything else, then we exchanges phone numbers and are meeting regularly and it's fun because conversations are easy, so he is the clothest thing to a friend I have here. But I have no idea how it would go if I need help in some bad situation, etc. because we didn't have any yet.
So yeah, this is kind of where I found my friends outside of school. Perhaps it can be some inspiration for you.