I think you are being conservative with that year range. It may not be 90%, but the first place I look for mechanical parts for my '33 is the local parts store too. I'd say I'm successful the vast majority of the time. So make that "1933 and up MoPar cars".
Sure helps if you know a semi-modern cross reference number though as their books don't go back that far.
For Plymouth, the 1933 model was a total redesign bringing in lots of stuff that was then used for many years after. So I don't know that you'd have quite the same luck with a '32 or earlier car. Wheel bearings and brakes would be about the same availability, but things like engine gasket sets, etc. for a '28 through '32 could be a problem at your local store.