Good luck...and I'm in a similar search: ATSF heavyweight passenger car decals. They're really hard to find anymore. After Champ closed their doors forever it seems like the obscure may be available only from the custom makers...assuming artwork can be found/created with reasonable effort.
My need, though, may be less urgent than yours. Perhaps putting your request into the Want-to-Buy section (Miscellaneous) of the forum may help? Also, I find that some of these sorts of searches are more fruitful among the 2-railers....they seem to have stashes and caches that have very limited awareness.
One last thing... If you come across a set of decals that are fairly old (as perhaps yours and my needs may yield) you need to proceed carefully. They will often need some restoration and/or preparation before use. And, sometimes, their best use will be as the artwork for having fresh duplicates created by a custom maker.
Just some thoughts... And our search goes on.....
KD