Bob,
I could not find any additional Lionel 15” PRR Heavyweight cars but you may want to do searches for K-Line 15” PRR cars. You may get lucky and find similar K-line cars. You could also look for MTH 60 Ft Railking cars. Even if they don’t match perfectly with the Lionel cars they didn’t in real life either so they should be fine as a addition to your train.
JohnB