There are always a few up on Ebay for about $50-$60. Your problem though as simonsays said is that you actually need 2 working fobs to create a third one. If you only have 1 or none, you have to go to the dealer or some place that has the ability to program the system from scratch---Plus you will have to get the car towed as you will not be able to start it. You can only program a maximum of 4 fobs, so if you do have 2 more working fobs, you will not be able to program any more.
Hi Dave. As previously mentioned, you need two working fobs to program another, so you will need to take your car to a Lincoln or Ford Dealer to have the new fob/fobs programmed.
And just to clarify what I understand Rongold to have stated above (sorry if I misunderstood Ron): Yes, you can only program a maximum of 4 fobs at any one time. However, when you program a new fob or fobs, previous fob programming is erased. That means when you have the new fob/fobs programmed, the old/lost fobs will be erased from memory and no longer work. So you can once again program up to 4.
This also means that if you purchase 2 new fobs and have them programmed now, then later find one or both of the two you lost, you can program those previously lost ones yourself (since you already have two working fobs). So you can then have 3 or 4 working/programmed fobs.
If you are contemplating only purchasing one, I would highly recommend you purchase two now and have them programmed.