I wouldn't exactly call him a 'significant other' but we met at a boat party. :)
My advice for single people who want to meet potential dates is to just live your life, have fun, put yourself out there, and try new things. Don't have the mindset that you're looking for someone to date because that can create a certain energy within you that makes you come off as desperate or needy for a guy. Just have fun, try new things, and make an adventure out of life then somewhere along the road, you'll find a guy you like enough to date :)
When I was your age, I was in various clubs on campus, I went to a lot of festivals, volunteered at galas, went for bike rides to the market near school, went out dancing, did a bit of modeling and that was how I met guys that I wanted to date. We would cross paths while I was being social and just living it up.
Originally I met her in a restaurant with a large group of friends but the day that changed her from being just an acquaintance and when I asked her out was during a Hanami party under cherry blossoms:
It's a chance (excuse?) to drink and socialize a lot with friends and acquaintances.
For your general question, I think the best way is just build up a massive army of friends and acquaintances with whom you regularly schedule events. And look to expand your army, adding more people to the group. If someone in your group makes a new friend, invite them to the group and keep making it bigger and bigger. You meet lots of love prospects that way and constantly have things to do every weekend.
University would be your best bet probably. I've been flirted with by cashiers at stores the most. Catcalled once, but Uni/clubs/bars probably. Though with clubs or bars, may not be the best way to find a keeper.