Where can I find rare and sought-after tubes for my guitar amp or bass amp?

For older guitar and bass amplifiers, rare tube types are often not only interesting, but simply necessary for continued operation if the original configuration is defective. These include, for example, 6C10 and 6K11 in various Ampeg circuits, 7247 / 12DW7 for example in the Ampeg B-15, 7581A as a robust variant in stronger 6L6GB concepts, as well as EL503 and PL84 in rarer European amplifiers such as certain Hohner units. Unusual types such as EL12 also appear again and again in older individual concepts and modifications. In such cases, what is needed is not just any standard replacement, but a suitable tube for the exact device and its circuit.