Shirokai make wide body kits for Mustangs, Camaros, Hellcats, BMWs, and SUVs. Many of which are in the current Forza games or have been in previous Forza titles.
Ford Mustang GT '18
Ford Shelby GT500 '13
Ford Mustang GT '05 (if it ever returns to the roster)
Ford Mustang SVT Cobra '03
Chevrolet Camaro ZL1 '12 (if it ever returns to the roster)
Chevrolet Camaro Z28 '15
Dodge Charger SRT Hellcat '15
BMW M2 Coupe '16
They have more vehicles and pics on their website