Use the Reference librarians at OKU:
They can help you not only locate archival collections elsewhere, but also the vast numbers of digitized sources that are already available to you. Don’t expect all of your materials to be available online, but if you are dealing with rare books, newspapers, serials, or other sources, you might be able to find them already digitized.
Talk to Interlibrary Loan
Even if archival collections are not digitized, heavily used or very important collections at most institutions are available on microfilm, which can usually be lent to Cornell patrons via ILL, or have single reels or some sections of reels sent to you on PDF. Olin has great microfilm scanners that can convert microfilm to PDF as well, so that you can save materials as needed. ILL might also request single items from archival collections. It is always worth a try.
Even if ILL can't get everything for you, if you have looked over a finding aid and determined that you really only need a few folders from a few boxes at an institution, or if your plan is to photograph heavily from a collection and then run back to Cornell to sort through it, it might not be worth making the trip. Many institutions have on-site scanning services that will photocopy a few folders for you at a cost. The cost is usually pretty high: you are paying for staff time to handle materials that are unique and often fragile and can't just be run through a photocopier. If there is a larger amount, you can also consider hiring an independent researcher to do your photographing for you. If you contact the repository, they might keep a list of contractors on hand. The costs vary from $25-$40 an hour, negotiable based on how much research skill is needed for the job.