Hi darietts,
Any normally open momentary switch will do.
Barn3 makes a tap switch specifically for TimeFactors: http://www.barn3.com/shop/tesla-tap
You can even get more comprehensive control from their OX switch: http://www.barn3.com/oxsystem
Other good options:
Saturnworks is a good option for a wide variety of switches: https://saturnworkspedals.com/product/tap-tempo-pedal/
I like the switches from Analog Endeavors: https://www.analogendeavors.com/category/aux
American loopers has switches of all types for Eventide pedals: https://www.americanloopers.com/products/mini-aux-switch-for-eventide-timefactor-modfactor-pitchfactor-space-click-less-two-button?_pos=2&_sid=0e363dc52&_ss=r
