If you want to buy enough pipe, theoretically you should be able to find a pipe mill that will make it to almost any dimensions you specify, provided that they have the technical capacity to do so. It all depends on how much you need and the mill's interest in tooling up to make it. Just be aware that you will need to get the same dimensioned fittings and valves. That might be far more difficult. Then if you can't get those, you will be stuck in special order custom transition pieces. You need to understand the full implications of using nonstandard sizes before taking the plunge. Life may not be easy and long lead times could easily bust your completion schedule. It is usually not worth the trouble unless you really need lots and lots of pipe.

For smaller quantities, you can only buy whatever the pipe dealers have in stock. The sizes they most likely tend to have available are those "common standard" diameters and scheduled wall thicknesses that you mention. If you do not want enough pipe to warrant a special mill order with custom dimensions, stick to 24", 30" or 36". That's usually the best thing you can do on many counts. That's the reason for having common std sizes.

First, the Pipe Codes are B36.10 for CS Pipe, and B36.19 for SS Pipe.

IMO, it could be not much market application for the 26" or 28" SS pipe, so they are not listed on B36.19. But, the MFR may fabricate any size of pipes for you, i.e. 26", 28", or >30", if really want it.