Take a look at the answers to How to find wife's maiden name in New England in the 1700s? -- my answer has general tips on finding records about women, and some of the other answers about finding female ancestors, or looking for the people's parents, may give you ideas about other record types to look for. The FamilySearch Research wiki on Pennsylvania has links to many major repositories and articles on several record types.
What searches have you tried so far?
For some of the people I'm researching, I was not able to find records about them which listed their parents, but I was able to find out by examining the records of their siblings. Does Elizabeth have any siblings?
Evidence about friends, associates, neighbors (or what some genealogists call the 'cluster') can also be useful for solving this kind of problem. You can see some examples on Elizabeth Shown Mills' website for her book Evidence Explained -- see her Quicklesson 11: Identity Problems and the FAN principle.
Edited to add: even if you can't find records for siblings, it may help to broaden your study and look at more of the local community. See the question How can I determine what records are available in a particular locale? What other resources exist for studying Amish and Mennonite communities? Look for general information about what records survive, whether or not they are indexed and available for a name search. A Google search reveals the Lancaster Mennonite Historical Society website and the Selected Church and Religious Genealogical Resources at the Indiana State Library. What other repositories might have records about the community Benjamin Miller lived in?
In really tough cases, you can't depend on being able to search record collections by the name of the person you are looking for. Even when the person you're seeking does appear by name, the name may not be indexed at all, the index may not match what the document actually says, or the name may not be spelled in the manner you expect. And if you don't know the historical context or the community, you may miss important clues because you won't recognize their significance. If you can find case histories of other people working in this area and around this time frame, read what they did and note what record collections they used, and what research methods they employed.