I've wonder also. have never seen any.
I think back of the grannys and mothers, There was never a chair or stool next there treadles, but they were not working with them, There isn't place to store a stool or chair. so those would be at the machine.
I do know my mom used a kitchen table chair, flat seat. I use what I think would be called a sewing chair, again flat seat, I would think a wooden stool like whats known as a bar stool would be correct,, The round seat and spindle leg type, some places still have those in a kit form