See comment here:
https://www.prototools.co.uk/suhner-collets.html
It may or may not be correct. The person writing it may believe they are telling the truth but it is highly unlikely they have done any research on the issue.
Dimensions here:
https://cnc-plus.de/en/Collets---Cl...UAL-Milling-and-Grinding-Spindle-Motors-.html
Given that many manufacturers make very similar types of spindles, look for the 'same thing' in a different colour and wearing a different name (possibly via AliExpress).
santool.de list many odd collet sizes, and give a more formal) DIN specification, so you could compare dimensions from above to see if they match anything.
Edit:
Just noticed tha the link is for the UAL spindle, not UAC. So you'd have to check that both spindles take the same collet.
See:
https://www.mk-produkter.se/app/uploads/2014/12/Suhner-Kat-E-DL-Motoren-slipspindlar.pdf
Mafell make a very similar spindle and talk of OZ collets. They are a standard item and have a DIN spec. That might be a good place to start - see if the dimensions of what you have correspond with anything in the OZ range.