I have one on my 306 W its one of the reasons I love that machine. They went out of manufacture for domestic machines when they stopped selling cabinet mounted machines. Back in the day when you could by a complete machine with the cabinet they were pretty much a standard, the foot control was also there but it was wrapped and up in the cabinet and you could take it down and use it. My knee control folds up if I want to use the foot control.
I asked my local sewing machine repair and he said all he could do for a new machine is mount the foot petal on a bracket so you could move your knee against the foot control.
Look for a vintage machine cabinet mounted and see of it still has its knee control.