Dear Home Ec 101:
Where can I get sodium percarbonate?
Short and Sweet
Sodium percarbonate is the generic name for the active ingredient oxygen bleach products like Oxo-Brite, OxiClean, or Clorox Multi-Purpose Stain Remover. When dissolved in water it creates hydrogen peroxide and soda ash. The bleaching action is accomplished by the hydrogen peroxide; soda ash is an environmentally safe byproduct.
You can find these products on the laundry aisle at most large discount stores. To get the most bang for your buck check the ingredient list and avoid those loaded with fillers such as sodium chloride (table salt). Oxyboost has a list of their distributors on their website and the products are available online.
You might want to check out the Home-Ec 101 Guide to Using Oxygen Bleach.
Good luck and happy cleaning.