I did some research on that for the IOS a few years ago. There is a manufacturer in southern Wisconsin. Google for relief mats or relief pads. Obviously you want an indoor-outdoor version that is slip resistant when wet. The company I found sold 4X4 foot interlocking sections. I'll google. Hold on...
Linear Rubber Products
5416 46th, Kenosha, WI 53144
p: 262 652 3912
http://www.rubbermats.com/\Humane Manufacturing
805 Moore St, Baraboo, WI 53913-2716
p: 608 356 8336 f: (608) 356 8338
http://www.humanemfg.com/ I liked products from both of these companies. Anything that will stand up to horse urine is bound to be tough enough!