Useful links:

Shops:

• Kitsch-Bent - Custom parts, mods, flash carts, etc.
• Hand Held Legend - Screen modification parts (backlights, bivert mod, replacement screen shields) Game Boy Builder
• Thursday Customs- Full custom builds, custom parts, mods, etc.
• Nonfinite Electronics - Refurbished systems, replacement parts, etc.
• Mortoff Games - Replacement parts, etc.
• Rose Colored Gaming - Full custom builds, Micro faceplates, GBA shells, & custom parts.
• J-Bit Retro - Full custom builds, shells, and screen shields (Facebook)
• General Painting Guide Covers most plastics used in the Game Boy line.
• EZ Flash IV - Flash cart for the Game Boy Advance
• Everdrive GB - Flash cart for the Game Boy
• Gamboy Mods UK - A shop for UK users with low shipping

Guides:

• Gameboy: Back-light Mod
• Gameboy: Back-light Mod inverted
• Gameboy camera tutorial
• Gameboy Color Internal Pro Sound mod
• Gameboy Color: Kitsh-Bent front-light Mod
• Gameboy Color: Horizontal front-light Mod

Reddit pages to follow:

• Custom Gameboys being added regularly
• Custom Gameboys being added regularly
• Custom Gameboys being added regularly and for sale

I will add more if I find any.