Where can I get the correct replacement emissions decal?

Less popular emission decals (like the ones for your truck and my car) are not reproduced. Best workaround I've found is to buy the closest one available, scan it in at high resolution, then rework the text and artwork to match what I need. Sometimes this involves scanning the artwork from the factory service manual. Then from there you just have to find an appropriate printer and decal material to print it out.

Year One's decals aren't their own; they buy them from others. The biggest selection of Mopar decals I've found has been from Osborn Reproductions; they have a lot of decals nobody else does. Their website is pretty bare-bones and there're no pics of the decals, but I ordered a bunch of decals from them last year and quality is excellent. Also see Performance Car Graphics. Another good source is ECS Automotive (you may want to call and talk to Dave Walden over there—if I'm not mistaken he can make repro emission decals to order if you don't want to take the project on yourself; he definitely has the materials and hardware to print out the decal if you send him the finished artwork). Looks as if Jack's also has a fair number of underhood decals.