Look for new mats at any of your local farm/feed supply stores.
Churumbeque is right, oftent he wood is part of your trailers structural integrity so plan on replacing it. After removing the old wood carefully inspect all the metal frame. While the sides might look good, years of horse pee can and will eat up the metal it's been running over.
My sister and brother in law bought a trailer with the thought they would fix it up. When they removed the wood they found the metal so badly rotted it wasn't worth the cost of repairing.