These look custom designed to me, or part of the design of the larger structure. I haven’t been able to find the exact setup in the photo, but you can buy the reflective film and attach it to a frame or surface that works for your setup.
Otherwise, you can get someone to cut some sheet metal (I’m guessing you’ll want it to be galvanized or maybe heavy aluminum, or stainless steel [if you have the $] for outdoor use) in the size and shape you want. The ones in the photo look like the irregular surface might diffuse the sunlight so it’s not as strong on the plants. I saw some concave discs, but I would think you should avoid those unless you want to fry your plants.