This list is mostly focused on useful supplier links for acoustic guitar builders.
Major Luthier Suppliers:
Luthiers Mercantile International, Inc.– Tonewood and custom instrument kits, tools, glue, etc. The Kit Wizard lets you buy whatever you want for guitar components (back, sides, tuners, frets, etc), yet get a discount as if you had bought a kit. LMI also has some custom made tools and jigs for side bending, fretting, etc.
Stewart MacDonald – A absolutely fantastic resource for parts, tools, and information. Dan Erlewine has many useful videos and guides – and has designed some ingenious jigs and contraptions for guitar making. Great stuff – but be warned – if you buy everything from Stew-Mac, you’ll spend way too much and will be missing out on some great options.
Other Luthier Suppliers:
Oregon Wild Woods – Tonewood
Kenneth Michael Guitars – Tools, kits
Waldron Music – Acrylic Guitar Templates
Canadian Luthier Supply – High Quality Radius Dishes & More
Caramillo (UK) – High Quality Bending Irons
Luthier’s Bench (UK) – High Quality Bending Irons
Luthier Suppliers – Radius Dishes, Jigs
DePaule Supply – Quality shell products for instrument makers & awesome pre-made inlays.
Bogdanovich Guitars – Info, kits, wood, and more for classical guitars. Definitely relevant to steel string as well.
Gurian Instruments – Bridge pins, End Pins, inlay materials, more
RC Tonewoods – Back & side sets, rosette squares, headplates and more. Recommended.
Woodworking and Shop Equipment:
Grizzly – Woodworking and metalworking machines. Maker of the popular G0555 14″ Bandsaw
Woodcraft – Woodworking tools and supplies
Highland Woodworking – Fine woodworking tools and supplies. Home of the Wood Slicer Bandsaw Blade.
Lee Valley & Veritas – Tools to lust over, parts, supplies, more
Lie-Neilsen – Very high quality hand planes and tools
Perfect Plank – Butcher Block slabs for workbenches, etc.
American Science & Surplus – all kinds of stuff cheap!