Mount Vernon Shops probably has them: http://www.mountvernonshops.com/Decals.html
Modeling the Bellefonte Central Railroad
Fan of the PRR
Garden Railway Enthusiast
Check out my Youtube Channel:
http://www.youtube.com/user/PennsyModeler
This will get you started if you are looking for the later PRR versions. Enough on one sheet to do all three of your cars.
http://www.microscale.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=MD&Product_Code=87-891&Product_Count=&Category_Code=
I don't have the exact timeline in front of me but as a rough idea, the late 1940s cars had imitation gold lettering. Then in the '50s & 60s it was Pennsylvania spelled out in the letterboard with Buff color lettering then simplified to just a keystone in the mid 60s to the merger in 1968.
http://www.rr-fallenflags.org/prr/prr-m647.jpg buff letterboard PENNSYLVANIA
http://www.rr-fallenflags.org/prr/prr-m449ags.jpg simple keystone
Hope this helps, Have fun! Ed
I picked up a trio of HO Con-Cor mP54s from my LHS at a very good price. They had been ordered for someone who never picked them up. They are unlettered, but in PRR paint scheme. Anyone know where I can get the right decals to letter these for the Pennsylvania RR?