I'm looking for digital data for Puerto Rico. Specifically census tract shapefiles from the 1990 & 1980 census. TIA
As referenced by Barry Carter's comment on the question, there isn't a straightforward answer to this question.
As described in this Census Bureau page, the entire US was not partitioned into Census Tracts until the year 2000. (In 1990, the whole country was covered by either tracts or the now-defunct Block Numbering Area, or BNA).
Those deeply interested in Puerto Rico may wish to read Understanding Census Geography In The Tiger Data Base: The Case Of Puerto Rico (PDF), a 1991 paper by Jonathan Sperling, of the Census Bureau's Geography Division. In it, he writes:
Reflecting the rapid urban expansion in Puerto Rico over the past few decades, the 1990 census shows that 56 of the 78 municipios are covered by census tracts compared with 22 in 1980 and only 8 in 1970.
which reinforces that, in Puerto Rico, specifically, census tracts were sparse before 2000.
Typically, for questions like this, I'd direct someone to NHGIS.org, but when I downloaded the 1980 Census Tract shapefiles (there are two versions), neither included Puerto Rico.
If NHGIS doesn't have it, you probably have your work cut out for you.