ERIC - Education Database
With any luck the secondary source authors will tell you what primary sources they used, which will make your job easier.
21st Century: Search Primo for online usage guides. One you're inside a specific guide, you'll need to search within the text for your rule.
Also try searching Primo directly for your rule and then limiting the results to "Reference Entries" to find entries within some of our online reference books.
The most current print guides are in the Reference department (4th floor, Wilson Wing). Call numbers: PE1407-1460.
20th Century: Look in the Main Stacks (5th floor, Reynolds Wing) in call numbers PE1407-1460. Some guides are scattered in other places, so check Primo if you have a specific title in mind.
19th Century: Check An annotated bibliography of nineteenth-century grammars of English particularly starting on p. 378 "Advice on good English." Once you have a title, you can usually find the full text in Google Books.
16th-18th Century: If you identify an older title of interest, you can usually find an online copy via Primo. (The bibliography of 19th century works mentioned above lists some 18th century examples starting on p. 357.)
Oxford English Dictionary
The OED attempts to find the oldest use of each sense of a word.
Pay attention to when the OED entry was updated! Compare the entries for hopefully and literally. We have older printed editions of the OED if you need to compare them. (Second edition. First edition.)
The link above works both on and off campus for you to connect your registration with the paid WFU subscription.
For other corpora, see the "Corpus Linguistics" tab.