Stores
Console5 - Parts and accessories
OGXStore - Xbox modding parts
Digikey - Electronic components
Mouser - Electronic components
Services
Redherring’s Mod and Repair Services
ModzvilleUSA's mod and repair services
Castlemania's list of recommended service providers for mods and repairs
Softmod Supplies
Compatible USB Flash Drive (Canadian Amazon)
USB Cable Adapter (for softmods) (Amazon)
Hardmod Supplies
LPC Rebuild Board for Modchips (OGX Store)
Conductive Paint Pen for TSOPs (Amazon)
Upgrade Accessories
StarTech IDE-SATA Adapter (US Amazon)
StarTech IDE-SATA Adapter (Canadian Amazon)
Custom 80 wire IDE cable (OGXStore)
CPU Interposer (N64-Freak)
RAM for 128MB RAM Upgrade (eBay)
3d Printed Fan Adapters (Retro Frog)
Controller Accessories
Replacement Breakaway Cable (Amazon)
Controller Extension Cable (Amazon)
Repair Parts
DVD Drive Belt Replacement (Console5)
Full Capacitor Kits (Console5)
Replacement Rubber Feet (Console5)
Other
AV Connector for DIY Component/RGB Cables (Console5)
Hawk (Open Source Xbox Live Communicator)
The Black Art of Xbox Mods by Jonathan Harbour - A now-dated book of Xbox mods from 2004.