Now I've received a second answer from Tektronix: The power supply for the DPO2000B series is not available at all, not even the 119-7266-00 for the DPO2000 series. They would not tell me if the power supply in the B models is the same. Note that the DPO2024B is still a current model.
I've just found out that I still have 8 months of warranty. I thought I had bought it more than five years ago ...
Thomas