Welcome back, Bob. We're so sorry for your loss, but glad you didn't curl up and croak as well.
To answer your questions (a bit):
Retreats are designed for fellowship. Folks who show up run the gambit from those of us who've been involved in this movement for well over 10 years, to those who just signed up, don't know nuttin', are scared to death we'll all turn out to be weirdos, but our curious. So far as I know, a good time tends to be had by all, and the fearful new tend to go away both reassured and enthused! SOMEtimes, "meeting" occurs. But I wouldn't attend with that as prime motivation and expectation. Contact the site administration for details on those coming up soon.
How long you gotta wait? Depends strictly on yourself. If your wife had cancer for some time, you knew this was coming, the two of you had plenty of time to grieve together and make peace with it, and you feel your grieving is done, go for it, and don't be ashamed to explain to an interested lady who asks. If she's worth pursuing, she'll understand. If she's more interested in conventions as defined by numbers than in relationship with YOU, she's not likely to embrace your family structure notions anyway.
Where ya gonna find 'em? The perennial puzzle. Main advice to offer is make a lot of friends, and when you find one or more you're interested in, make them REALLY GOOD friends before taking it too much further. Having said that, as the friendship is developing, be very, very open about your beliefs and long-term aims. Nothing ANYBODY hates worse than to be courted into friendship and caring under false pretenses!
Good luck, Bob,
Cecil