How do costume designers educat themselves culturally and historically on clothing?

Hi, Mallory,
Historical clothing is often a class that you must take when completing your degree for costume design., so reviewing some of you old notes can help. If it's something that requires more research I have found going to the experts for advice (i.e. Going to the Tribe if you are doing something about native American's). The internet can also be helpful. I found living history groups to be a wealth of knowledge for their time period speciality. Rental house are also helpful when looking for costumes for a time period.
I hope this is helpful
Gabrielle