Do you mean these ones ? They are intended to keep pressure on the rubber seal to keep rain-water out. They are held in place with tiny sheetmetal screws with a tapered thread - much like a wood screw.
If you don't have any, you can make them out of brass shim stock about 0.015" or 0.020" thick.
If you mean the "L" ones that screw the the bottom corners of the frame together, they are available from Moss.