Well, one of my current partners, lovefromgirl, was single when we met, so it's definitely possible, yes.
If I may ask, why does your husband have the requirement that she be single? Are there certain attributes of a single person that he cannot find in someone that already has a partner, or what?
Where to go? Out. It's as simple and as complex as that. He needs to meet people. bets way to meet poly-minded folks is to get involved with your local poly group. Not everyone will be an instant dating prospect, like any cross-section of folks, but it at least increases the chances. If online dating appeals to him (it doesn't to quite a few folks) then OKCupid is a good place to at least find people. I'll warn you, though - as a male the odds are stacked against him. There are lots of guys (and couples) looking for single women - just scan through the personals boards on this site to see that, so he is going to have to stand head-and-shoulders above the rest to find someone.
If he can work out why he needs single, and can maybe find those qualities in someone that already has a partner, then the odds are going to go up dramatically.
What happens is he finds a single lady and then she finds another boyfriend - is it automatically over between them?
"Listen, or your tongue will make you deaf." - Native American Proverb