The Magical Yellow Book
(NOT Harry Potter!)
If I could only have one catalog on my exhibit resources/supplies shelf it would hands-down be the big yellow book from McMaster-Carr.
The widgets and gizmos they sell have gotten me out of many an exhibit jam. I love the fact that you could buy a railroad car wheel here if you wanted/needed to (and get it delivered the next day!)
Check out their searchable website, but try to get a copy of the paper catalog to peruse as well.