My lfs still has a rather large new old stock batch 10mm and 15mm U and elbow, the 15mm U are sold out
(i need them).. He an oldtimer from past generation and says it's the last stock and no longer in production, even doesn't remember the maker. He bought a large batch of stuff at an auction many years ago and there is so little demand he still gots them today. I believe i'm the only one buying them off him. I guess they are relics from past time when diy in aquarium was rather an obligation instead of an odd dying out skill. He has more silly stuff left i never knew it was ever made.
But they are PVC and as Kippersarnie says indeed not perfectly glass clear, not that is needs to be, the hose also isn't..
For diy you could try to use Acrylic tube and put it in an oven, than heating up can easily be regulated. I do not know the exact working temperatur but theoreticaly acrylic gets pliable from 80°c to 110°c.. Acrylic tubing doesn't have an iso/din standard and can be found per mm in diameter. Whit clear PVC it can work the same, but needs a little higher temp. And available sizes are rather in iso/din 16,19,25mm etc. There are also silicone solid tubing available which has a much higher melting temp, which you could use as internal guide preventing kinks instead of sand.
Anyway best changes of finding omething like this ready made, scavange Aliexpress and or Ebay.. If it still is made, it is in Asia.