Where can I get suitable replacement tubes for historic radios and pre-war devices?

For historic radios and pre-war devices, it is especially important to find the tube series and version that matches the respective unit. Older series such as RE, RES and RENS are particularly sought after, supplemented by some early A-series tubes. This applies to many classic units from Telefunken, Saba, Mende, Blaupunkt, Lorenz, Philips, Graetz or Loewe. Especially with such radios, a suitable and carefully selected replacement is more important than just any tube, because construction type, base and technical design must match the respective device series.