Idea 1: Troll google for personal letters of reference with a search engine - here are some samples:
- include the strings "letter of reference", "to whom it may concern"
- exclude the string "template"
- filter for PDF and DOC files
examples results pdf1, pdf2, pdf3
include degree (Master, PhD)
include department (Physics, Law)
The downside is you'll have to do the categorization yourself
Idea 2: (ethical?) Post fake job advertisements on Craiglist or Indeed and ask people to send CV with letters of reference. At least one would then have geographical and topic-based categorization.
Idea 3+: There are plenty of less-ethical things beyond here. That being said, analysis of the hiring process is an important research topic and leads to important discoveries like - Are Emily and Greg More Employable than Lakisha and Jamal? A Field Experiment on Labor Market Discrimination.