I suggest maybe checking back at the same location for a couple of weeks. I have seen this happen a few times where an orphan speaker shows up on the floor, and then its mate appears a few weeks later.
Blows my mind how it could happen, but I also saw an orphan speaker at one location, and its twin at another location within the same thrift store chain!
Ask the manager or some friendly worker. It worked for me. I found a lone JBL Apollo C51 one week, and I asked if there was a match, but they said they didn't think so. But when the manager later saw the other one she held onto it (they know my face from being there all the time), and the next time I went in I got the other one.