I first encountered McMaster-Carr in 1978. The engineering department said if you need anything get it from the big yellow book! If they don't have it - no one does!
So I said,"Oh yeh! Do they have sling psychrometers?"
They had three models!
[Only two today!]
I got to visit one of the old warehouses in the '80s and the wooden floors had deep grooves worn between the shelves. No automation - everyone knew where stuff was at like the old-time hardware store!
I still go there first and if they have it - order it! Like MM says their service is REAL!