need parts

I am trying to find the small white plastic ( 5/32" x 1/2" x 3/4") guide piece that is fastened to the frame that the locking slide bar moves up or down on activated by the locking lever that draws the window shut & locked. There are 4 of these guide pieces on the frame to keep the locking bar in place in a vertical alignment. I work in maintenance, in a 60 unit senior housing facility with lots of windows and I now have 2 windows that these plastics guides have broken free from their rivets.