A decal company could make you either stencils to paint with or decals of the letters you want to put on. Thats what many body shops do for graphics they paint onto cars. I'd call your friendly local body shop guy, if you know one, and ask for a reference. If not, try your mechanic or maybe the yellow pages for a sign company or graphic design company. If you talk to three or four people eventually you'll get to the right persons.

If you just want standard stencils you could try an art supply shop or architecture supply place or even arts and crafts store.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi