Parts with"28" SERIES 385 CHANNEL BALANCE 2740

I am ordering two “28" SERIES 385 CHANNEL BALANCE 2740 on your web site.  At the same time, I also need to order two plastic parts which are on the top of the window channel and screws which install the parts (please see attached 3 photos).  Nevertheless, I could not find them on your web site.  Could you tell me where to find them on your web site?  If it is not available on your web site, could you tell me where else to order them?  Thanks and I look forward to hearing from you as soon as possible. Bob