Seattle area soaring?

hello! my name is kevin and i am trying to get a schedule to have my dad go up in one of evergreen soaring gliders. he has wanted to ride for as long as i can remember. he lives in north marysville and frequently goes out to the airport and watches. but every time i go on the scheduling site for a ride for his birthday they are always full. i know it's volunteer basis but can i get a idea when someone would be available to take him up? please let me know
kevin wahlgren