Search:

Sorry, not wanting to overpersuade you ;-)
But they do have the smaller ones as "Type A" with 30mm hex and in stock. That E20 is actually the one I have. Somehow I thought E25 was mentioned earlier...

Ah found the place where I bought them: https://www.arceurotrade.co.uk/Catalogue/Collets/ER-Collet-Nuts-Wrenches-and-Spanners/ER-Collet-Nuts-with-Ball-Bearing-Type-B
Pretty good deal for GBP 8.16...

I am somewhat sceptical about the 100 hour rule (other than overly proactive maintenance). There are no moving parts in operation so why should they wear out? I would rather believe if somebody said...

I had that problem occasionally with long 1/2" diameter bits (i.e. 6") when the vibration and leverage worked the nut loose over long hours.
I have since replaced all my collet nuts with the ball...