How about a 3-position toggle/rocker switch with one position being on, one position being off, and the third position being momentary-on?
Momentary-on is indicated by either parentheses "(on)" or abbreviated as "Mom On" or "mom-on".
Amazon has
these "Round Paddle Lever Momentary Toggle Switch - SPDT / On - Off - (On)" switches.
Sigma switches lists "1N= SPDT on/off/mom-on" rocker switches -- no idea who stocks them, though.


Another possible option is something like a
59071 Mom On-On-Off double pole ignition/start toggle switch, depending on the application.

Scott