Take your existing water filter cartridge with you to Lowe's, Home Depot or where ever. They are used to this, people do it all the time.
They will have several brands, types and prices. The mechanical fit is really all you have to comply with. The details are then up to you and your budget.
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2008 (or is it 2009?) Rockwood 8280SS - 2007 F-150
Lot's of mostly Kenwood radios