Where can I find replacement tubes for measuring instruments, tube voltmeters and oscilloscopes with tube technology?

For measuring instruments, tube voltmeters and oscilloscopes, many tubes are mainly sought for preservation and restoration. This applies, for example, to classic Tektronix oscilloscopes such as 535, 545 or 545A, to Heathkit tube voltmeters such as IM-11 and similar VTVM models, as well as to laboratory instruments from HP such as the 410B. What is needed here are not the picture tubes / CRTs, but the other functional tubes in the measuring and amplifier sections. Typical tubes in this field include, depending on the device, 12AU7, 6AL5, 6DJ8 / ECC88, 6CL6, 6CB6, 6DK6, 6AU6, 6U8, 5651 or high-quality types such as E80F for precise measuring and amplifier applications. In the Heathkit IM-11, the combination of 12AU7 and 6AL5 is explicitly described, and in the large Tektronix 545 units, 6CB6 and 6DK6, among others, are typical tubes of the vertical amplifiers.