you can get the clay at an arts store (i.e. "Michael's" around here). The Potash was difficult to find, the big stores (Home Depot and Lowes) don't carry them. I finally found some at a local nursery - you can do some 'phone shopping' beforehand. The dolomite can usually be found at an LFS that has reef supplies.
The mineralized setup works great for my nanos (2.5 gallon setups), going to try it on a couple larger ones and try to set up a large, high-light system like Aaron has going.
A caution here.
Most of the "clays" - all of them that I found locally except one loner product, actually - in stores like Michaels and Joanne's were not true clays, but man made polymer materials. When you see the self setting, etc, odds are it is polymer. Be very careful of what you are buying in those kinds of shops. Not that it'll do harm, but you want the iron content of real clay, which isn't in the polymers!
I went to a pottery supply store to get a block of real, pure clay.
For Potash, call around to Farm and Family, John Deere Supply, type stores. It is a pure form of a fertilizer used for farming. Bag will say like 0-0-60 or 0-0-50.
FWIW - All my tanks are going to mineralized soil. I personally feel it is every bit as good as Aquasoil, etc, and in some ways, better!
I've even begun to test the limits of those plants that are not supposed to fare well in it. Happy to be able to report that Tonina Fluviatilis is growing gangbusters, Belem is growing as well. Blyxa Japonica and Aubertii also doing well. Erio Type 2 is struggling. I slaughtered Ludwigia Pantanal in the past, need to try that one again. All plants that supposedly don't fare well in this substrate system (due to lack of water column dosing).