I would try a nightclub that allows 18+ in. The crowd will be mostly young kids like you so you’ll feel more comfortable, but will run into the odd older lady.
I’d just find a place you like, and go out there a few times a week and ask around to see if there’s a night when they tend to come in more.
Bars are better, but you might not be allowed in as young as you are. But the thing about bars is, a lot of them will look the other way if you can pass as older than you are and you get to know the staff. So that’s an option worth considering.
Any bar is going to be way better than a club for sheer quantity. But at clubs you’ll probably find that the older ladies there will be into really young guys.
How do you get them interested in you? Just talk to them. Ask them if they like younger guys. Ask about what they like in bed. Don’t be surprised if they drag you out of the club.