Regional Gliding School (Central) operates out of Picton and Mountain View; can't get much closer than that!
Of course, that won't do you much good if you are too old for the air cadets, or want to learn soaring rather than basic gliding. You can find a complete list of soaring clubs at the
Soaring Association of Canada website (click on "SAC Services" and then "Clubs").
The club nearest you is probably
Rideau Valley Soaring. It is not exactly close to Belleville, though.
