Using Raspberry Pi to Control a PWM Fan and Monitor its Speed - DriftKingTW's Blog
PWM Regulated Fan Based on CPU Temperature for Raspberry Pi : 4 Steps (with Pictures) - Instructables
Amazon.com: Retroflag NESPi Case+ Plus with USB Wired Gamepad Controllers & Cooling Fan & Heatsinks for RetroPie Raspberry Pi 3/2 Model B & Raspberry Pi 3B+ : Electronics