Fitting non OEM tubes is a risky business. I have seen cases where a 3rd party fitted a tube and the equipment experienced nothing but trouble as the spec was not exactly the same.
Most x-ray equipment is under full contract with the manufactures who would react very negatively to the fitting of non OEM parts. To the extent that if an non OEM tube was fitted any subsequent faults, on any part of the system, would not be covered by the contract!
Also who would take the legal responsibility if there were some sort of incident that led to a court case?
Saving a little money in the short term on a 3rd part x-ray tube may cost a lot more in the long term!
If budgeting is an issue then take out suitable tube contract to avoid sudden and large outlays in the event of a tube failing.
It could be argued that the 3rd party tube suppliers are promoting cut-price x-ray tubes to line there own pockets! Remember if the equipment manufactures can't make reasonable profits there will be no products to buy in the future (or a reduced number of suppliers reducing the competition).
All in all, like lots of great ideas for saving money they turn out to cost more in the long run!