Hello and Welcome to Budget 101, MissM1240! Good to have you here. Enjoy!
Any questions, just ask. (Washing Soda also known as Soda Ash and Soda Crystals is Sodium Carbonate (Na2CO3). Commonly used as a water softener and in recipes for homemade laundry detergents, it can usually be found in the laundry section of the grocery store; although, some people have a hard time finding it in their area.
Another source is a pool supply store.) Not sure which recipe you may be looking at as there are numerous ones, the links that AngryLioness mentions are in this article https://www.budget101.com/do-yourself/super-laundry-sauce-faq-4214.htm. Hope this helps. Thanks; Virginia