Sears is definitley not going to have any decals for any tractor from that time frame. Don't even waste your time with e-bay since most of them are incorrect and look like crap. I do not know of anyone that sells restoration decals for the 18. Mine were mostly complete when I restored mine and I had to have them duplicated off of my pics. Although, I know someone that does good reproduction decals if you provide him with good digital photos of your old ones. He can edit them, and print new ones out on a solid backing.
I'm going to have the larger yellow and orange Sears decal that was on the top of the hood reproduced.