I used to go regularly to meetups in my local town that were specifically about meeting strangers. They would usually be advertised as for people 'new in town'

I used to go regularly to meetups in my local town that were specifically about meeting strangers. They would usually be advertised as for people 'new in town' but in reality a lot of people went there just to hang out even if they'd lived there for years. Because the events were about nothing in particular, you had to learn how to make smalltalk and then how to nicely escalate it to more interesting conversations. Obviously it wasn't truly random strangers because people self-selected by going, but there was a steady supply of truly new people who just wanted to meet others and make friends. Met lots of people that way, made some great friendships and even business connections. Will echo jemmyw about flights. Obviously you don't want to be the annoying seat neighbour, but usually if someone wants to chat they'll make that clear fairly fast. I had some great and interesting conversations with people on planes. Even got numbers from a couple of girls :)