I was surprised to recently hear from one pilot that the best place to get the add-on rating in California is in Hawaii. http://higlideracademy.com/ No smoke. DPE on staff. Winch and Aerotow. Sustainable flight off of 1000 AGL tow/launches. Five mile ridge adjacent to the airport. Thermals over the pineapple plantations. Total cost including airfare and accommodations was comparable to training on the mainland. (Caveat: this person rented a car and 'car camped' across the street from the airport).

I've also heard good reports about Williams for add-on ratings.

Keep in mind that if you want to actually fly gliders, you will want to take additional training beyond the add-on rating. But doing some concentrated training to get started is highly recommended. The add-on rating will 'make you a better pilot' even if you don't pursue gliding, and training in glider is great fun!