Washing soda alternative sources:
Washing soda can be bought in bulk cheaply under the label "soda ash" or "sodium carbonate" at farmer's coops and at pool stores.
Or you can make your own washing soda if you can find a cheap source of baking soda.
There are many ways to get cheap bulk baking soda. At a feed store a 50# bag is only a few dollars (it may be labelled as sodium bicarbonate). Or you might find a good sale on boxes at a grocery store, cheaply marked boxes at a salvage food store, and so on.
So let's say you have found a cheap source of baking soda. All you have to do is bake it to turn it into washing soda. Heating baking soda releases molecules of dioxide, which converts it into soda ash (washing soda).
But save on energy too. Rather than heat the oven just for this, the next time you are using the oven for baking, have a cookie sheet ready with a layer of baking soda spread out over it. Don't make the layer too thick. A half inch or so is enough. If it's too thick, the dioxide can't properly escape from the lower layers.
After you finish your baking, leave the oven on, slide the cookie sheet in, and bake it at 450* for 30 minutes.
That's it! Now you have washing soda.
FYI: Arm & Hammer also makes their washing soda from baking soda. Both products start out as sodium bicarbonate mined from the earth. So when you buy washing soda, you are buying what started out as baking soda.