I found CDS tubes in 20mm and 26,9mm diameters - maybe not ideal, but close enough
And there is hope that I can buy 2,5m of 20mm and 4,5m of 26,9mm - not whole 6m...
I need to ask in one more place in my city - one of the biggest dealer in Poland, but it's definitely wholeseller, so it can be difficult.
Thanks Alga - I will check your sugestion - maybe that's another option.