@zebra, a little bit lasts a long time. I cannot even remember where I got my pack from. But Techden sells some, Benibachi mikronecton or tantora montmorilonite.
http://www.thetechden.com.au/Tantora_Montmorillonite_Powder_50g_p/tmont50.htm
Our other fav site sponsor Newbreed Aquatics also sells a variety of sizes (pssst, and slightly cheaper) https://newbreedaquatics.com.au/shrimp-additives/benibachi-powders
I don't know how much you need, but you can get 500gm packs.
Or the 1kg pack in stones (and crush into powder yourself).
Ignore the product description of "old sea mud", they don't mine this from the bottom of the ocean.
i don't know of any other bulk sources unless there is a quarry somewhere.