The Reference Collection is housed on the 2nd/Main floor of Malpass Library. Books in this section can't be checked out, but can be used within the building, scanned, copied, etc.
The general section on historical costume and fashion is in the GT500 range of call numbers.
Reference works provide authoritative, general information on a topic. They are a good starting place if you don't know a lot about your time period or location. They can also provide clothing vocabulary for you to use when searching elsewhere.