Where does the funding in business school PhD programs typically come from? My classmate is in a prestigious business school PhD program - stats / operations research.  He told me that they have no teaching duties, and they aren't allowed to do any external internships.  The business school's Dean has been furious about letting their PhD students intern in the past and then losing many students early after they land lucrative full-time offers from industry.  So apparently, the stipends are increased to make up for the new rule that the students cannot seek internships before finishing their PhDs.

Where does the funding typically come from for business school PhDs?

For reference: United States.
I don't know the answer for sure, but here's a few things you can try that will probably uncover the answer:

First off, go to the school's website and look at the pages for current or prospective PhD students. Usually this will explain what financial support is offered, and possibly where it comes from -- e.g. departmental fellowship (coming from the department's budget), or research assistantship (coming from faculty research budgets).

Look up the current PhD students. Scan through their websites and CVs for any fellowships. Then look at their papers. See if their papers acknowledge funding sources. All of my papers from my PhD student days have a footnote thanking support from a National Science Foundation (NSF) grant and this is indeed what paid for my studies.
You can also look up some of the faculty that have PhD students. Do their websites/CVs acknowledge any funding?

Lastly, business schools tend to get a lot of money from MBA students. Harvard's MBA tuition is $73,440 per year and their current enrollment is 1,859 students.  Ignoring financial aid, that's a revenue of $136,524,960 per year. They can definitely afford to support a few PhD students! I suspect that this is the main funding source.