First: Excellent Model! I just received some decals from K-4 supply in HO scale for a similar project. What kid of variety did QA&P have for boxcars? Any prototype pictures available?
One photo came from the internet...the other is a scan from the Hofsommer book The other posts I have done here indicate I often print my own decals. I use MS Publisher to do the art work and save it as a .pdf. Included here is a .pdf of the old QA&P herald which can be printed on waterslide decal paper.