In California, CARB rules (California Air Resources Board) for replacing catalytic converters are a bit tricky. You may still be able to get the OEM part from Ford, but for my 2007 4.6L Limited the cat has been discontinued and is no longer available. There's a list of CARB-approved third-party catalytic converters for your particular year and engine size here:
https://ssl.arb.ca.gov/AftermarketParts/catalysts
Ignore the first few that have EO "Withdrawn", since they are no longer approved for use in CA after a certain date.
You have a couple of choices of products. "Direct Fit" are bolt-on parts that include the dual converters and connecting pipe, but because they replace both left and right banks and are designed to be direct replacements for the original parts, they are more expensive. "Universal" cats are sold singly for driver or passenger sides. (I think for our vehicles they are the same part on both sides, but not all makes and models of vehicles are symmetric this way.) These parts require cutting out the old cat and welding the new one into place, which can be done by most CA-approved muffler shops.
I needed to replace the driver-side catalytic converter, and ended up buying one off the CARB database list directly from the manufacturer at millercat.com for $900. My muffler shop says they'll install it for $250, which was the cheapest solution I could find. (Another shop had the same cats in stock but wanted $1800 per side installed.)
For those who live in states that only need to meet federal EPA guidelines, replacing a catalytic converter can be done much more easily and affordably.
