Just have a look at various Ebay sellers & check their feedback prior to ordering. Don't buy any mats that are thinner than 17-18mm thick as they will possibly rip under the weight & twisting & turning of a horse on them in a stable.
You should be able to buy 6' x 4' x 18mm at about £25 or less per mat + delivery. Check the delivery price as this varies alot due to weight of the mats (they'll probably come on a pallet). For a 12' x 12' stable you'll need 6 mats....so 6 x £25 = £150 + delivery. Fitting isn't hard, you'll need a tape measure, straight steel edge & a stanley knife. Oh & careful of your fingers