Try your nearest Ford dealer.I worked at a dealership for years and they always had them up in the Custom Shop.Along with every other type of decal related to a Ford vehicle.If they didn't have it,they sometimes could order it.You could also try Pep Boys Automotive.You may also check www.50resto.com and mustangsunlimited..along with texasmustangs.There's a vast assortment of aftermarket mustang parts retailers online.
Advanced Auto Parts used to carry decals similar to that.Not sure if they still do.
Sorry to hear about your car.Been there already.Lost my decals with the re-paint.