2005-up wheels will not fit a 2004. The hole pattern is the same, but the offset is different by over an inch.
Ford made the 18"x8" rims for your truck only one model year - the 2004 model year. There were two different styles available, King Ranch and SVT (Ford Racing Special Vehicles Team). TireRack had some left until recently, but they're no longer on their website, so I suspect they're all sold out now.
If you can't find them at a wheel dealer such as Discount Tire, then they should still be available from your Ford dealer. But not cheap.
My Sierra Blanca in the sig pic was a great pickup for 11.5 years. I sold it a coupla years ago. I drove a hand-me-down 2003 F-150 SuperCrew 4.6L 2V for a while, but it was unacceptable for towing more than a rowboat. Replacement is a 2012 F-150 EcoBoost SuperCrew Lariat that tows my 5,000-pound TT like a dream.