Do you want specifically custom shifters or would COTS transmissions work? If you want COTS, you could look into both the
AM Super shifter and the
VEXPro BallShifter. I think there's also a video or two floating around YouTube that explains ball shifting. Never bothered looking for dog shifting. The CAD for both products are on their respective pages, so if you'd like you can download them and take one apart without actually buying the gearbox.