Washing soda is sodium carbonate.
I purchased 1 lb of the sodium carbonate from nutsonline.com under the baking category. I know some people are paranoid (Armour Hammer as washing soda), I was too. Just in case you want to share with other Earthclinic readers.
After many weeks of trying to hunt down Sodium Carbonate from local stores, I found the beer and wine making supply store to carry "Soda Ash" which is the same thing and costs very little.
Sodium Bicarbonate (baking soda) can be found at local stores but you can also find a natural or organic one in some better markets for a little more money. Not sure how much less chemicals and/or metals is in the latter but I can tell you it tastes much better and disolves easily compared to the most common household baking soda.