The type of stablizer depends on the project. There are about one million different types. Wash away, tear away, cut away, different weights, sticky back, iron on, etc. it's a whole topic unto itself.
I get mine at all stitch:
http://www.allstitch.net/department/....cfm?killnav=1
If you scroll to the bottom of the page in the link, there are several articles that explain the differences. Because you are going to run an afterschool pgm, I would pick a first project that will be made with a firm woven fabric, like weavers cloth, or pre made aprons from Sam's club. A medium weight tear away will work well. I would stay away from knits at the beginning. T-Shirts can be tricky.