Widely available cassette cogs: recommendations? I'm trying to determine what are the most widely used/available cassettes and corresponding cogs. I don't care much whether they are high-end components or low, though I expect the low-end ones to be most widely available since they will be found on low-end production bikes sold in the thousands. Durability is of greatest concern to me, with things like weight being a decidedly secondary factor. The chains in use on the wanted cogs will be 3/32.

Background on my question. I know how these cassettes are constructed and have taken them apart, having done quite a lot of bicycle mechanicing in my lengthy bicycling career (of over 50 years). I also know the economics of buying full cassettes as opposed single cogs: I'm prepared, if economic concerns so dictate, to buy whole cassettes to get just the cog I need from each one, though I don't like the idea of just chucking the rest of the cassette into the garbage. 

I have a very specialized project for which I will need a reliable source for cogs for some years to come. The project involves further modifying the complex drivetrain of my custom-built recumbent tandem trike (delta). My idea for the modification will involve using single cogs at either end of the jackshaft on this bike, in place of the cassette/freewheel that are now attached there. The gearing will instead be moved to the rear drive wheel, possibly to an IGH hub like a Rohloff (the only IGH hub, it seems, rated for tandem use). The cogs I will be needing will probably be in the 22-26 tooth range, though I still need to do some more detailed calculating to determine what size will be optimal for this project. Obviously, I will be needing to replace those cogs periodically over the bike's lifetime, so having a ready supply for some time to come will be important.

Input on widely available cassettes/cogs (brands, models) will be appreciated.

P.S. I need the cogs to be completely flat to attach them in the manner I'm envisioning (they will bolt on rather than sliding over a splined central cylinder), so I'm ruling out freewheel cogs, which have a bit of a lip around the threaded hole via which they attach to the freewheel body. I'm located in the U.S., btw.
11-28 through 11-46 cassettes are all pretty ubiquitous these days.

Sprocket sizes for a few cassettes in this range:

11-12-13-14-15-17-19-21-23-25-28

11-12-13-14-15-17-19-21-24-27-30

11-12-13-14-16-18-20-22-25-28-32

11-13-15-17-19-21-23-25-27-30-34

11-13-15-17-19-21-24-27-31-35-40

11-13-15-17-19-21-24-28-32-37-42

Just based on how common they are, I'd say that choosing somewhere in the range 12 - 28 teeth will be safe. However, larger sprockets are attached to a spider and some smaller ones have a built-in spacer. Fundamentally there are probably many issues with choosing to use cassette specific sprockets. You might be better off looking an single speed or internally geared hub sprockets, which do exist in non-freewheel versions for 3/32" chain as well as 1/4".

Another alternative is to look at the small end of chainrings in common bolt patterns, 22, 24 and 26 teeth are pretty common. There are plenty of 3x7, 8, and 9 groupsets around for inexpensive bikes and that is not likely to change soon. Chainrings are usually flat and can be purchased individually.