I was googling on uses of borax when I came across a Gardenweb forum. I don't know how old or new it was, but everyone was lamenting that washing soda was virtually unobtainable in the US. Because I wanted to help (and other reasons) I have just joined Gardenweb (It's great, BTW) and have this to say:
Washing soda or sodium carbonate is widely available everywhere. You have to look for soda ash, found in swimming pool retailers or the swimming pool section of any hardware shop. It is the same thing.
I live in Australia but I'm sure washing soda or soda ash will be found in the same manner in the United States