Hardware, home improvement centers, restaurant and janitorial supply stores, nursery, or on line.
At a big box home improvement store in their garden department or a plant nursery (greenhouse) company.
Garden stores stock it usually.
Nurseries, but mine never has it. I purchase it online from Amazon
Amazon carried it
I bought mine on amazon here https://www.amazon.com/dp/B00VVMTM14?aaxitk=oUuI6mc4z3P2GdhX4P0pZA&pd_rd_i=B00VVMTM14&pf_rd_p=44fc3e0f-4b9e-4ed8-b33b-363a7257163d&hsa_cr_id=9311760160501&sb-ci-n=asinImage&sb-ci-v=https%3A%2F%2Fm.media-amazon.com%2Fimages%2FI%2F71F6Ji2clZL.jpg&sb-ci-a=B00VVMTM14
A garden center will have higher concentrations.