Uhhhh, I think I must have transposed some numbers. Should be 25.4mm to an inch.
That said, i don’t know about and 23.5mm stainless. My mild steel tubes at 1 inch, 0.06 wall, were about as stiff as 3/4 emt (23.5mmOD).
I’d recommend a local metal shop if you have one unless you go with emt, then any home improvement store should be fine.
Ummm… No. I don’t. But I know there are those who have used such things around here. You could probably do a forum search for “water cooled spindle”, and find a few examples fairly quickly.