StewMac isn’t the only catalogue, nor the oldest, but every luthier and instrument repair dude I know has one on a shelf by his/her bench. I’ve been getting the catalogue for 6-7 years to find and order replacement and upgrade parts. I buy instruments at garage sales and fix them up to give away or use (I play a bunch of lute-family instruments — own about 10 guitars, a couple of banjos, a sitar, a bandurria, a ukulele).
StewMac carries some really unusual tools and they have the widest selection I’ve seen: fret files, bridge plate iron, inlay routing jigs, saws for mother-of-pearl and abalone, bushing pullers. They carry specialty glues, hard-to-find woods (Sitka spruce, koa, curly maple, Indian rosewood), laminated paua abalone, pre-sawn wood blanks for mandolins, banjos, ukuleles, and guitars.
If you’ve busted a hole in your guitar, you can also get the repair supplies, tools, and instructional books/videos to fix it for less than the guy at the shop will quote for doing the job. StewMac put in a minimum order of $30 not long ago, but I still think it’s a fair deal.