Can you buy a modern throttle and cable for a TY250 with original Teikei carb?  The only cable I see available is for the original throttle.  Or do you buy a Domino or Amal throttle and make your own cable? 

Are cables somewhat standard between carbs such that if I move on to a mikuni or OKO would the same cable work?  Are other carbs an improvement over the Teikei?  I'm not seeing Teikei jets or parts available so it make be best to change anyway.       

I've made a number of throttle cables from scratch using bicycle brake cable and homemade brass ends attached by soldering.  It's easiest to start with something close and modify it.

In the States, Flanders sells a variety of stuff to build your own cables: https://www.flanderscompany.com/collections/cable-parts

The D slide OKO provides faster response in both directions than any round slide carby.

Venhill and others make suitable throttle cable kits.

Yes different carbies require different length inner cables, unless you are very lucky.

The important Teikei carby parts for the carby that came on the TY250A have been unobtainable for many years.

The Domino twistgrip is a good thing. I have broken many AMAL T200 twistgrip cable attachments in crashes. The Domino is more crash-resistant and is lighter and you have a choice of pulley sizes. The Yamaha TY250A and the later TY250 twistgrips are also easily broken and the cable loop catches on things.