IN the UK they usually sell them for the Diesel models - like the 1.4L / 1.5L - to help retain heat in the wintertime - else they don't get used - or not really needed, for USA unless you want to run with one - they sell kits that contain the insert - fasteners and the shield - and some even offer a cutout to access the oil filter.
But, with that kind of mess and the weather we have - they really are not needed unless you use these exclusively on dirt roads or Alaska where extreme cold can affect the ability of the engine to stay warm or keep some level of efficiency.
Now, also - bear in mind that Fiestas are "low profile" vehicles so having a splash shield is a quick way to wind up with plastic strewn about behind your vehicle after running over some railroad cross-bucks - it can make a mess of the undercarriage if you use short low-profie tires to reduce ground clearance.
I was locating some in UK sites but backed away when I saw how these can "block" access to your undercarriage if you needed to get at the radiator or the oil filter for any reason - it can use up to 6 fasteners that have to be unscrewed to drop the guard, and if you saw how "thin" the plastic of it was, you' would not be too impressed with the "felt feel material" it is and for durability purposes - I would want more of a metal cage similar to what Pickups and SUV's would use to protect their lower oil pan.