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If you are talking about the small light assemblies that twist into the bumper, that's a junkyard or dealer part, in my book. You might be able to track one down online but by the time you have it shipped to you the final price will probably be at or near what you'd pay for one at the dealer. A junkyard would be ideal.
FWIW, I needed the same part one time and I got it from my Ford dealer, the parts guy told me they had a whole box of 'em in back. I don't recall the price being outrageous.
I have had good luck with Rockauto.com. Everything that I have purchased arrived exactly as advertised. They do carry many many Motorcraft parts. I purchased an after-market electric rear view mirror which worked out well.