There's Daox of Ecomodder.com who 3D-prints a 1, 2 or 3-button shifter knob:
For sale: Custom 3D printed shift knobs with integrated buttons - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
With the Ecomodder logo
For sale: Custom 3D printed shift knobs with integrated buttons - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
With the Ecomodder logo