The Davis Square Community

I'm going to make my own laundry detergent, for which I need some washing soda. (Arm & Hammer Super Washing Soda, seemingly the only brand that exists anymore.) I can buy it online and have it shipped but that seems to defeat some of the purposes of making my own detergent. So I was hoping that the collective wisdom of the DSLJ crowd could help me out with a local-ish source.

I have a car, so I'm willing to drive but I'd like to know it's there before I haul my ashes out of the city limits. And calling doesn't seem to be the solution -- I have had very little luck asking folks on the phone. (The conversations are nearly Kafka-esque. "No, not baking soda, washing soda. No, it's not a beverage, please don't transfer me to that department!  Not detergent, to make detergent! NO, I don't need a HAMMER!")

Thanks!