I'm looking for a crosswalk between MCDs (Minor Civil Divisions) and PUMAs.
They are called equivalency files and can be found here: https://www.census.gov/programs-surveys/geography/guidance/geo-areas/pumas.html
The Bureau equivalency files don't include the new PUMAs yet, only the ones designated in 2012.
MCDC's Geocorr application can create crosswalks between many different types of geographies.
We call MCDs "county subdivisions" in these apps.
For current (2020) MCDs and the new PUMAs, choose County Subdivisions as your source geography, and PUMAs as your target geography.
I hope this helps.