May have already posted this, - if so, I apologize. After responding to ads and placing our own ads in swinger's pubs, we abandoned that approach - it's time consuming, and often yields guys who aren't what they claimed, no-shows, dick pics and so forth. It can work - we met one guy out of hundreds of responses to our ad seeking single or married males for threesomes.
1. What worked best for us was: friends that we knew already, - more than one guy from my wife's work who had already not so subtly indicated his interest. When her boss came on to her, it was a gift that paid off for us for several years. So we'll call these guys coworkers as well as friends. And the big benefit of this approach was that it was my wife choosing who she'd be interested in. Not me, pushing her.
2. If you live near a city with a lot of business centers, try the bar at a higher end hotel used by business people traveling for work (and smart locals). Go as a couple between the hours of 6 pm and 11 pm. It's all horny guys who have had a drink or two and who have a room there at the hotel. Hubby needs to drift away after you get there, so that guys can hit on you. Hubby might wait elsewhere in the hotel - even get a room for you to come back to after your tryst if you don't yet want to go home. Sometimes you have to spend a little, to get a lot. This isn't a time to cheap out. A room at a cheap motel sends the wrong message.
3. As others have noted, businesses (like bars, clubs, and even restaurants) near military bases. These are also good for meeting couples.
Bars can be pretty noisy, making it difficult to talk, but it's something you just have to work around. Body language plays a big role. That's how we met or got to know men for my wife, along with one chance encounter when she was dropping the kids off at ...... care. She met a guy there who was doing the same. Two weeks later, he was in our bed with my wife. It's one of our favorite encounters that lasted for more than one time.