When you have an idea about a subject that requires accurate historical information, it’s not always easy to know where to search. Here are some tips that I learned. They could be useful to you. The documents come from several types and are not all in the same place.
People houses:
Letters, intimate correspondence. Testimonials from the era or descendants (stories passed down within the family or heard)
Archives: (national archives etc.)
Medical reports
Minutes
Birth certificates
Period newspapers
Theses: (specialized websites, universities etc.)
History student theses
Books :
Current authors who have written on or around the same topic
Local fund:
Public Library