406 V6 wrote:They are supposed to last up to 5 years, together with the rest of the hydraulic system before maintenance. That's the reason why they are only found on the Citroën network, unlike older spheres that are sold easily

I think its simpler than that -
If they last as long as the multilayer spheres on the Xantia then they will easily last 5 years without losing significant pressure, and since the C5 only first came out in 2001 the oldest ones are only just reaching 5 years old now, so previously there would have been no market for 3rd party C5 spheres !!
The only C5 spheres that would have been replaced until now would be those that failed early due to some kind of defect, and many of those would have got replaced under warranty.
Why stock them if hardly anybody needs any yet ? The market for replacement spheres doesn't exist until the model reaches an age where the average sphere is expected to need replacing... and since the C5 uses a new design of sphere and a new hydraulic oil it is a radical departure from early sphere designs...
Give it a few years and C5's will be reaching the age where their spheres actually do need replacing in large numbers and suddenly there will be a big market for 3rd party C5 spheres and people like GSF will start stocking them...
Regards,
Simon