Taken off a new bike, Shimano 9071 Dura Ace one-button Di2 shifters, includes e-tube wires. These shift the rear derailleur only, one button shifts the rear derailleur up, the other down, customizable with the e-tube software. $110 shipped.
https://www.competitivecyclist.com/shimano-di2-tt-remote-switch-sw-9071?skidn=SHI0626-ONECOL-ONESIZ&ti=U2VhcmNoIFJlc3VsdHM6OTA3MToxOjE6OTA3MQ==
https://www.competitivecyclist.com/shimano-di2-tt-remote-switch-sw-9071?skidn=SHI0626-ONECOL-ONESIZ&ti=U2VhcmNoIFJlc3VsdHM6OTA3MToxOjE6OTA3MQ==