Yes, 20 Mule Team Borax is (according to the box behind me on my photo chemical shelf) sodium tetraborate decahydrate.
I've heard a lot of discussion and debate whether Arm & Hammer Washing soda is anhydrous, monohydrate, or decahydrate of sodium carbonate -- given that sodium carbonate is usually used in unbuffered developers, generally with a warning against acid stop baths, it seems it should be safe to use some excess; assuming that it's decahydrate and applying the correct conversion would ensure you have enough alkalinity (there are few, if any, cases where you want the alkalinity to have low reserve).
As someone mentioned, Red Devil Lye is 100% anhydrous sodium hydroxide (though it quickly absorbs water from the air when opened). The same is true of other brands of plain lye (sold as a drain opener and for making soap), and they're usually cheaper.
I've recently read of someone making para-aminophenol (aka Rodinal) from acetaminophen, aka Tylenol. Folger's Coffee Crystals can make a nice, easy, cheap staining developer. Ascorbic acid is available in tablet form, or as a powder from health food stores (be sure to adjust formulae according to ascorbic acid vs. sodium ascorbate -- they have different molecular weights and different solution pH). The commercial canning product Fruit Fresh is mostly, if not entirely ascorbic acid, also. As previously mentioned, commercial stop baths are not only cheaper than white vinegar (by the time you calculate dilution), but the indicator lets you reuse them to exhaustion instead of using them one-shot.
I've heard of people getting sodium sulfite from canning suppliers; it's used as a preservative in making sausage and canning meats; the food grade is perfectly fine for photographic use. Sodium metabisulfite (called for in small quantity in some developers and fixers) can be found at home brewing suppliers; it's used in wine making. One might also be able to obtain phosphoric acid from brewing suppliers; it's used in some home-brewed specialty drinks like sarsaparilla, in colas, and for making "phosphates" sodas.
One might be able to obtain technical grade sodium thiosulfate, aka hypo, from dyers' vendors; it's used as a setting or mordanting agent in dyeing wool (thought I doubt it would be significantly cheaper than photographic grade hypo from a supplier like Photographer's Formulary or Artcraft Chemical). Adding ammonium hydroxide (plain household clear ammonia is a 3% solution) to a hypo solution would produce a simple, slightly alkaline rapid fixer, though the ammonia odor might be enough to cause problems.
Citric acid (odorless stop bath) is sold in many grocery stores with canning supplies. As mentioned above, boric acid can also be used as stop bath; it's sold as an eye wash in drug stores (though it's a lot more expensive than either citric acid or white vinegar). Boric acid powder is also sold, at considerably lower price (technical grade instead of USP), as an ant and roach killer (white powder you sprinkle around baseboard) and as a flea killer (sprinkled in carpets). And of course a simple water wash to prevent carrying developer into the fixer is actually sufficient for film unless you're using a very short developer cycle and need a precisely timed stop for consistency.
