I bought one from Zeigler Ford a few years ago for my Fusion V6 Sport. They had the best price at the time, but the plans have gone up a lot in price since then. I used some FordPass reward points to help pay for it. You definitely need to shop around, and most have online calculators for cost. Several online sellers, mainly Ford dealers who sell in bulk to sell cheaper. Definitely buy the official Ford ESP to save the hassle. It is like insurance, waste of money if you never use it but worth the money if you do need it. That being said, it has been a waste of money for me as I have had zero issues outside the standard warranty, and only a couple small problems within the first three years (I guess that is a good thing). With the problems I have heard with the new Explorers, sounds like it might be a good idea to have it. Have been considering it for my new used ST.
Be aware that the plan length in years includes the factory warranty length and is not added to it. Also, you can buy a plan any time, but if you buy it after the factory warranty has expired, it may be required that the vehicle be inspected by a Ford dealer, and the plan price is higher. Prices do increase occasionally (that bit me in the butt when I bought mine), so best to buy sooner than later. If you sell the vehicle or decide to cancel the extended warranty, you can get a prorated refund, but have heard you don't get as much back as you think you would. I'm guessing they figure in the factory warranty years on the prorated amount, which lowers the refund. I believe if you sell the vehicle, you can transfer the remaining warranty to the new owner.