Stewart MacDonald – This is pretty much THE website for luthiers. You’ll find just about anything you could possibly need to build an instrument here.
Guitar Kit World – I’ve bought a few kits here. You can customize your wood, hardware and things like righty/lefty and they’ll include things like sanding sealer. Little pricier but you get what you pay for.
Seymour Duncan Wiring Diagrams – Wiring an electric instrument is actually not as complicated as you might think if you’re new to it. I’ve used SD’s wiring for everything I’ve built.
How to Build Electric Guitars: The Complete Guide to Building and Setting Up Your Own Custom Guitar – This is a good guide on how to build bolt on guitar kits. You’ll learn about finishes and relic’ing instruments here
How to make a DIY Guitar Kit play like a high end custom shop kit
Pickguard Templates– Want to cut your own pickguards? Here’s some PDF templates
Guitar and Pedal builders group on Facebook – Community to explore and share ideas for building guitars and pedals