Todd,
Welcome to the forgotten side of O-Gauge trains. Almost all of my O-Gauge tinplate decals had to be custom made. I use, www.lbrenterprisesllc.com Their work is top shelf all the way. The same holds true for my Standard Gauge decals.
If you do find a size and style of decals you like, you can have them duplicated for your own use. I've never had any problem doing so.
God Bless,
"Pappy"