Image via KBD fans
Plates hold the switches and in some cases the stabilizers in place. They come in a variety of materials. Aluminum, brass, carbon fiber and polycarbonate. You can opt for a plate-less design where your mount the stabilizers and switches to the PCB directly. I personally prefer the plate because it gives the keyboard more structure and it makes the switches feel less wobbly.
Make sure your plate is compatible with your Keyboard's layout and PCB.
Plates are usually sold with the keyboard group buys. You can buy plates from most keyboard manufacturers.
Links:
Thocstock - For In-stock parts
Mechgroupbuys - They track keyboards group buys