People love to suggest groups and classes and stuff instead, but in my experience that's one of those things that sounds like it would work, but isn't actually effective in practice.
The problem is if you go to them just to meet girls you come off creepy and its really no different from going to a bar except people actually into the thing get annoyed at you for treating it like a bar (rightfully). So it has to be groups you genuinely enjoy for the activity and not trying to meet women. But if you go to them because you enjoy them you'll probably end up in groups that are not primarily single women and you'll want to keep going to the same ones repeatedly because you enjoy them so you don't get a bunch of different chances.
Even if the groups don't skew particularly male, a class or group might have a dozen people and half of them are dudes and at least half the women aren't single and half the single women aren't people you are interested in in the first place so you end up with like maybe one chance per class/group to ask someone out. And since its a group you like you aren't doing it once, you are doing the same group for a while so this gives you maybe one chance to even ask someone out every few months and the odds that your only chance that quarter is someone who's into you back and that the two of you hit it off on a date are low.
You could try yoga or something that skews female, but most of the same issues still happen. Its weird to keep going to new random classes and start hitting on people, but if you go to a regular yoga class repeatedly you don't want to be known as the guy who hits on all the girls so you get like maybe 2 shots at it instead of one. Its still not really the long term solution to meeting people we wish it could be.