After checking several locations, all I found was Arm and Hammer super washing soda (So Clean variety). I called the toll free number, and apparently it also contains some added 'fragrances' and 'sulfectants' whatever those are. Where would I find pure sodium carbonate? Chasing these things down can be a real hassle.
Cheers,
Crisp

Place Pure Sodium Bicarbonate (Baking Soda) in a pan
cover it with some tin foil but not too tight (to allow vapors to escape, and to prevent it from bouncing all over the place)
heat for ~ a few hours @400F until only about 2/3 remain.
This is now Sodium Carbonate. *applause*

yes as post above, you can make it from sodium bicarbonate... or you can try finding it in hardware stores/pool equipment shops, sold sometimes as 'pH Up' for pools (be sure to read the label)

You can also put your Sodium Bicarbonate in a pot on the stove top (high heat) - it only takes about 5-10 mins depending on the amount used. Beats waiting hours for ovened Carbonate.