You can make your own Washing Soda for mere pennies! It is basically Baking Soda that has had the water cooked out of it. You can find Baking Soda at your local Aldi store for about 49 cents per pound.
Pour a 1 lb. box of baking soda on a cookie sheet. Spread it out evenly. Bake in a 400 degree oven for 1 hour. Stirring 2 or 3 times during the baking process. Let it cool and store it in an airtight container so the water vapor in the air does not convert it back to baking soda.