l-o-n-g main slide for old Besson Eb comp tuba

[Rumblebuffin]

Hello,
Looks like you could do with a high-to-low pitch conversion slide from an old Boosey or Besson. EEb ( These are common in the UK because of the large numbers of brass band instruments converted from high pitch in the 1960's)
As these are typically between 4-6" longer on the slide legs (And sometimes even more!) you'll have to shorten it I expect.
(FYI The better quality conversions often had a 'donut' loop on the narrower slide leg.)
As a lot of these older instruments have worn out, some of the larger repair concerns may well have one of these slides in their scrap pile.
May I suggest Rosehill instruments of Beaconsfield or Mcqueens of Manchester as a starting point as they might be able to help.