I think they're generally referred to as "marquee retainer brackets". Pretty much every cab used different ones, but there are some generic ones out there. Out of my two cabinets, only one uses a bracket (the Atari one). The other uses a routed out edge to hold the plexi. Works quite well.
If you have the Happ part number, you might check with Ponyboy or Divemaster over in the B/S/T forum. They can probably get it for you and it usually costs less than ordering directly from Happ.
Using the drop ceiling rails is a good idea. I'll have to remember that for the future.