I've been looking, and I do know of one. It's quite a challenge but I'll be looking for new places. It's best to go on Sundays, I believe all quarries are closed that day.
I know of a couple in the GTA. I've only been to one and didn't have much luck, based on Google maps their best mud spots were very close to the main buildings which were lit up with lights so I didn't want to get too close. Found a spot further away on the outskirts of the quarry that was about waist deep. Still on the hunt for some better spots though!