Place drumsticks in a slow cooker lightly sprayed with cooking spray.
In a small bowl mix together the tomato sauce, soy sauce, brown sugar, and garlic and pour the sauce mixture over the chicken. Cover and cook on low for 5-6 hours or until chicken juices run clear. (internal temp. of 185F)
Remove the chicken to a serving platter and pour the juices into a small sauce pan.
In a small glass mix together the cornstarch and water.
Skim any fat from the surface of the sauce, bring the mixture to a boil, and then stir in the cornstarch mixture. Stir and cook until mixture thickens.