LEGO Technic Gear shifter ring kit (axles and gears) 15 pieces : Amazon.co.uk: Toys & Games
Amazon.com: SEEMEY 184PCS Gear and Axle Set for Technic Parts Compatible with Lego Technic Parts, DIY Gears Assortment Pack((Liftarm, Pins, Axles, Connectors)) for Technic Building Blocks Set(Random Color) : Toys & Games
Bricker - Pièce LEGO - 6543pb01 Technic Stick Shift Plate with '1' and '2' Left and '3' and '4' Right Gear Shift Pattern (Stickers) - Set 8880
LEGO MOC LEGO compact 4-speed paddle-shift gearbox by BrickbyBrickTechnic | Rebrickable - Build with LEGO