The Model D graders were built from 1949 to 1965, using the WD/WD45 engine. The Model DD (Diesel) were built from 1954 to 1965.
These are sometimes hard to find as landscapers usually pay premium prices for them as they are more suited for landscape work than large graders.
Recommend that you check with dealers in the upper midwest. I remember seeing some in Billings in the early 70's that might still be running.