check out your local hardware store/chain. And do not just limit yourself to the metal tube isle. I know that Bunnings (Australia big chain hardware store) have 25.4mm tubes in their metal area…..
But I found they also stock Stainless Steel tubes in their Bathroom (not plumbing) isle. Yet if you use their search portal and search for tube 25mm, you only find the 25.4mm tubes. If you add the correct key board then you get the one’s from their bathroom isle.
The root idea of MP-CNC is think out side the box on making a CNC, well when looking for 3D printing parts/CNC/milling to use, look out side the obvious isle in the hardware shop.