I would check iboats first, if not then you can pull the pads and take them to the local auto parts store and get them to match them up. Most will have a set of pictures, in a book or on a computer, they can measure and match to. If you need hubs and rotors I believe that Champion has their own website.
You need a much better picture. If you are wanting to replace the rotors if they are worn, you need to figure out who makes the brakes. The easiest way is to take a good picture of the caliper assembly and someone can identify it. Or, you can call the trailer manufacturer and ask them what they put on it.
Sometimes, its just as inexpensive to buy a whole new assembly for both sides.
If the rotors aren't gouged, don't turn them just to remove rust. It will disappear the first time you apply the brakes. Wait to have them turned until you do a complete brake job. The thinner you make them the more they will heat check. Or possibly crack.
