For MTB:
https://www.tufo.com/en/mtb1/?pl_art_gsegr5=1588 (they also make 26 for artistic cycling)
https://www.a-dugast.com/winkel?Coll...inbike&lang=en
I believe Challenge made some as well though couldn't easily find it on their website
For Road:
https://www.vittoria.com/us/en/tires...tires/juniores
Dugast also makes them for road bikes but 26" was not a common size unless talking 650c which was semi common)
I am sure others make them for road uses as well I want to say Continental did it and probably others I just cannot recall.
However
Ironfish653 had some good advice minus the "they just don't exist" part as they do they just aren't common..
Nino Schurter rode tubulars for a while and of course won so there is some provenance there. Though honestly unless I was a professional racer I would not want 26" tubulars and really wouldn't want tubulars on anything but a more dated rim size (I am purely talking 559 ISO here because the others are super dated) just seems silly as finding tubular rims is super tough and tires aren't hard but aren't easy to find. Better off with clinchers or tubeless assuming of course you actually wanted sew ups.
In the future though if you are looking for help you should post as much info as you can so we can actually help you. There are many different 26" sizes and you could be needing it for road or MTB or something different and maybe again you aren't looking for sew ups like IF mentioned.