ProQuest Dissertations and Theses Global has full-text coverage from 1743 to the present for many dissertations in the United States and from around the world. Many graduate students are required to deposit their dissertations here or in another database (see the list on this LibGuide page for other places to search).
The best way to access this resource is to use the QuickSearch Databases list. Once there, search for the name of the database. (Searching for "ProQuest dissertations" will also work.)
I have searched for Seneca grammar using the free-text search. Note that many irrelevant results are in this search because the language shares a name with a famous historical figure. To narrow down to more relevant results, use the subject filter to limit to Linguistics. This will improve the results set.
After limiting the search, many of the results are about Seneca and other Haudenosaunee languages. While they are about aspects of grammar, we need to think about the words we are using in our search.
If we change our search to "a grammar of" AND Seneca AND language, the quotation marks will search the phrase a grammar of; the Boolean operator AND will let the database know that we want the words Seneca and language included. It's useful to find common phrases in titles of grammars to narrow down one's search. We could also search for the phrase "Seneca grammar" using an OR operator, creating this search: (("a grammar of" AND Seneca) OR "Seneca grammar") AND language.
Without picking a specific language, as long as we make sure we apply the Subject filter so we are limiting the results to Linguistics, one can also search for the phrase "a grammar of" AND language, which will (usually) show us documents that are grammars of various languages.
We have done all of this with the basic search. If we used the Advanced Search, we could limit our search to identify words in titles, abstracts, and so on.
This search for a grammar of the Tuscarora language retrieves one result.
ProQuest Dissertations & Theses (PQDT) provides a very comprehensive collection of dissertations and theses and is the official digital dissertations archive for the Library of Congress. The database provides full text for most dissertations after 1997, and older materials that have not been digitized are available for purchase as printed copies.