You will not find a supplier for these retainers. This is not an extruded knock-down kit. The retainers, as well as the returns, were made in the shop with a sheet metal brake. I've made hundreds of these signs. In the old days, they were formed of 16-gauge Paint-Lok steel or sheet aluminum in a similar thickness, probably .040". We cut 45-degree angles on two of the four sides with snips, leaving the other two sides straight-cut. The angled cuts overlap the straight cuts for clean-looking corners.
If the retainers are held on with screws, remove all the retainers and bring them back to the shop for repainting. If there is any latex paint on the retainers, sand it all off. If the top and bottom retainers are riveted to the cabinet, sand and repaint them in place, as jburton said. You should be able to bend them back in shape by hand if needed. The corners should tightly overlap so they fit cleanly, and the tops and bottoms should mate up at the middles. The tops and bottoms probably overlap a little at the middles so there is no gap. Use extra screws as needed. Sometimes a screw through the face of the retainers at each corner keeps the corners held together. Use new screws. I prefer truss head screws for their low profile.
This job may not be worth braking up metal for new retainers.
Obviously, if you are remaking the sign and changing the lighting, bring the whole thing back to the shop to work on it, or have a new cabinet made. You will need two people to drop it to the ground.
There is probably a frame of 1-1/2 angle iron on the inside, with the returns riveted to the angle. That's how we made them. It's a cheap way to make a lighted sign, but we made hundreds of them this way and we made them fast. A cabinet made of an extruded aluminum knock-down kit will look nicer.
And I'm sure your layout will look far better than the crowded, amateurish layout on this sign. It's an eyesore.
Brad in Kansas City.