Tube vendors.

All the above are excellent sources for tubes
Richard Gray , my tech geek swears by Amplified Parts, so thats where i now get my 12 power tubes, 4 sets of trios, Not so easy to match, which AP does their best to fulfill,. RG says they have excellent  testing equipment to match. 
My next set will be EH KT88's. 

Matching is a bit of a shell game. The tubes are seldom run and matched under real world conditions which vary mightily between amplifiers. A match at one point may not at another.

For a well matched set, they should be run for several hours before matching.

Amplified Parts doesn’t have a single mention of matching as related to tubes on their website, so quite possibly are selling sets matched by the original tube vendor.

June 27, 2024.   I just had a very pleasant experience with Brent Jessee Recording re: the purchase of a matched pair of NOS 45 tubes for my Alan Eaton mono blocks.  Brent very clearly states his warranty is 30 days.  I bought the tubes and within a few weeks went out of the country for three weeks.  When I returned I turned on my system and after a few days realized I had one dead channel.  One of the 45 tubes was dead.  I sent the tubes back to Brent so he could match up the good tube with another tube from his stock.  To my pleasant surprise he returned a new matched pair at no expense to me.  Brent Jessee-highly recommended.