Have you checked on ebay? I have gotten ones for my 2010 (it uses band type) and they worked great. Yours are attached to the valve stem. I believe you will also need a tool to re-set the sensors. I also bought that on ebay. Ford's systems lack the information and ease of re-setting like the GM TPMS. One of the few things Ford skimped on.
TireRack also sells them but they will be more expensive. You might also ask your installer if he can get them for a fair price. I paid around $25. ea for the valve stem TPMS units for my G8. The re-set tool on ebay for Ford was under $30. Only issue I have had with it was this fall when it had set in car and was like 30 deg out. Battery was flaky but it is almost 4 years old. Once warm it worked fine.
