I understand that you might be a little put off by the rather frank and pointed feedback you have recieved so far. However, you indicated that you were doing your research and looking for advice as you were unsure of what happens next...
It just so happens that what you and your husband are looking for is not at all uncommon. There are lots of people out there looking for a third, usually bi-woman to add to a pre-established couple. Hell, my wife and I were very much the same. What we found was that finding a single woman to meet those expectations was exceedingly difficult to find. (Not impossible...we know people where it's happened...but it's vary rare that it works out that way) And the more specific or unusual your arrangement is to begin with, it reduces the possibilities even further.
One thing about the forum here is that people here generally won't lie to you. They will give you the raw unvarnished truth as best they can...and sometimes that's a disappointment to the fantasies we may be holding about the future. Often that truth will be blunt and unforgiving of the fantasy.
As for the judgement you may be feeling...no one here knows you, or your situation as well as you do. They can only run on the information you provide. So if you're not providing all the info, they'll fill in the rest with assumptions. And if you carry on with your research, here or elsewhere, you'll find out where a lot of these assumptions are coming from. Google Unicorn Hunters anywhere and read some of the stories, and you'll see why. As I said, there are many couples seeking a third. And unfortunately, there are plenty of testimonials from the thirds who have tried out these arrangements and been burned by the experience.
It has nothing to do with your fantasies, or intentions...but there is a lot of cynicism about it in the community...and be it on this forum or elsewhere in the poly world, that is a challenge that you'll need to deal with if you carry on with the path you've chosen. My best advice is to take the time to understand the experiences of the people who have tried this... both the couples, and the thirds, both the good and the bad to get a better understanding of what things might work...and more importantly what landmines you can avoid...and do it before you bother prospecting for anyone, because walking into a situation without proper preparation decreases the chances of becoming one of the success stories.