You can make washing soda from baking soda by yourself in the oven. The only difference chemically is the presence of carbon dioxide. Google "making washing soda from baking soda." It's a simple process and the chemical change is easily evident.
Patricia is right, you can't replace baking soda for washing soda, it is very different. You can replace the Nels with Zote Soap, however. It comes in large pink and white bars and is the laundry aisle of many grocery stores. It is very good for hand washables also. It is inexpensive.
As for the washing soda, besides Walmart, you can buy it at Kroger Grocery Stores and is probably available at home improvement types (Ace Hardware, Lowes, Home Depot) stores also, I haven't actually checked on that in a while, though.
Washing soda is very much different than baking soda. The two do not inter-change well, (trust me I know, I tried it the first time I made detergent). However, you can use:
Dawn dish soap (the blue, original kind)
Anyhow, what local grocers do you have? I know Wal-mart has these items. However, in my small-town area, the local grocer carries these items all in the laundry aisle, but sometimes if you ask a manager the stores can and will order you these products. It's worth a try! Good Luck!
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