knobs for locking window arm

My home has Pella windows from about 1966, if they are the originals. They have an arm that pushes out and pulls in to open and close the windows. When the window is closed there is a little knob on the end of that arm that fits into another piece of hardware which then locks the window tight to the frame. They do screw in to a hole in the end of this arm. Several of mine have stripped since they are actually just plastic. Any such part still available? Thank you.