I‘ve not researched Mary Bowser, but the Library of Congress holds extensive primary source documentation on Harriet Tubman. A good start: https://guides.loc.gov/harriet-tubman/digital-resources. You’ll note on that page, there’s an “Ask a Librarian” link on the left - helpful for your specific questions. I go to the LOC first, because over the years local museums and libraries have digitized their collections and sent remaining extant documents there for archival storage. Then the digital versions are made public through the LOC. For example, here in Loudoun County, VA, the slave censuses were once held at The Thomas Balch History Library, but a few years ago the contents were digitized and the books sent to the Library of Congress).

Because Harriet led 300 slaves into Canada, primary source documentation can be found there as well: https://guides.canadacollege.edu/africanamericanprimary/undergroundrailroad