Plymouth fours are the superior engine, I would think... they were only made for a few short years, but numbers were high enough for some to exist.
I can only speak for Australia, but I know of a few kicking around here. I'm sure you'll find them if you look in the right places.
Dodge 4s were made in even bigger numbers, I'd say (over more years to start with and an established make by the twenties), once again I know where I could put my hands on one or two here, so they'd have to be able to be found in the US.