Mark,
Do you want standard-type TZR carbs with angled bores (i.e. TM28SS) or non-standard TM28 straight-bore carbs, as found on Suzuki RG250s (and some RM80s)? If you want the straight-bore TM28, these are available as aftermarket products from Mikuni dealers (
https://www.mikunioz.com/shop/tm28-418-28mm-mikuni-tm-carburetor/). These can be converted to cable-operated choke/starter, but the standard TZR parts won't work.
If you want the standard-type TZR TM28SS carb, I have never seen these available from a Mikuni dealer - I think they were only available as genuine Yamaha parts (or Kawasaki, as they were also used on the KR250). However, I think you would be extremely lucky to find complete NOS carbs nowadays!
There were a few Yamahas that used TM28SS carbs, including:
TZR250 1KT-based models (including 2XT)
TZR250 3XV (R and RS models)
TDR250 2YK and variants
R1-Z 3XC (actually TM26SS, but can be modified)
DT200R 3ET-based models
DT125R later models - not 100% sure which ones
TZR125 later models - again, not 100% sure which ones (not the Italian-built 4DL)
The above models used the same basic carbs, but all had subtle differences.
Also of possible interest is the TMX30, as used on DT200WR (3XP), WR200 (4BF) and DT230 (4TP). The TMX30 is also available aftermarket -
https://www.sip-scootershop.com/en/products/carburettor+mikuni+tmx+30+_40070000 (there is also a near-identical TMX27 available).
I'm not sure whether your engine (and more particularly your air-intake system) is standard or modified, or how much effort you want to make to adapt non-standard carbs. The easiest option would be to obtain a set of good used carbs from any 1KT-based TZR, or from a TDR. Other versions may need a bit of jetting work in order to make them compatible. Are your existing carbs unserviceable, or are they missing altogether?
Regards,
James