The latest firmware is 4.3.8966 as to a link, it depends which Touch you have. Which touch do you have? is it an A, B, or C? If you don't know, try a sync and see what you get. If you do know which Touch you have, use the following site to get the link to side load the firmware.
https://geek1011.github.io/KoboStuff/kobofirmware.html