Unrelated… guess who finished her wedding story???

Every case is a little different. Some fic writers get betas by soliciting or mentioning their interest in a beta. Other fic writers get offers. Or if you’re buddies with other writers in the same fandom, you can swap fics and beta each other. The best betas for your fic are your readers, who are already invested in your work. Look for readers whose comments stand out, if they have good constructive feedback to offer and/or are just very nice, message them, cross your fingers, and see if they’re interested. It worked for me! :)