Another vote for Brent Jessee, for signal (octal and nine pin) tubes (excellent tube quality/testing protocols and customer support*).
For power tubes: Upscale Audio and these guys:
https://tctubes.com/6550-KT88-KT90-KT120-tubes.aspx (*ditto, for both)
This eBay seller, in Germany, for the real thing in NOS, rare Siemens, Teles, etc: