5650396003 Rondelle

I have a string trimmer model ST151OS that has a washer missing. The washer goes around the nut that goes from the motor to the trimmer string head. I took the the string bump head off to replace the line. When I took off this bump head and pulled out the housing with the spring attached to it, the washer that was inside the shaft area that the spring on the bump head rides on, fell out and landed somewhere in the grass. I couldn’t find it. I have gone to the hardware stores and tried to replace it, but they don’t have a size that fits it. I’ve contacted power plus parts 3 times and tried to order the replacement washer. 3 times now Power Plus Parts has sent me the wrong washers. It has cost me $26 so far and still don’t have the right part.