UseYourNoggin, could you provide a little more info about this spacer, do you know the year and engine size of the Fusion, also the thickness of the spacer would help me narrow down the results.

 

You'll have to search for one on-line or your local performance shop. The Mazda V6 3.0 ltr is basically the same engine as the Ford Fusion so they interchange. That one will probably not fit on your 2.5 ltr 4 cylinder. Thickness is around 3/4". Another gasket and longer bolts are also required, and they do come with it. I am not sure if they even make one for the 2.5 ltr.
Good luck in searching.
If you change the O2 sensor, just change the one before the pre-cat (other one after pre-cat does not calibrate air/fuel mixture).