You don't necessarily have to visit an archives in person to use their physical collections. Many archives offer reproduction services--photocopying or scanning--for collection materials that are not available online. Learn as much as you can about their collections and policies from their website. Then contact the repository's staff, who should be able to help narrow your search and advise you on the best approach to getting the information you need. Note that many repositories charge for reproduction services.
It takes a lot of effort for archives to put their collections online. In order to be accessed in a meaningful way, images and documents need to be scanned one at a time at high resolution; have descriptive information attached; and be individually uploaded to an interface that makes it easy for the public to sort through the materials. All of this takes a great deal of staff time, and professional scanners, server space, and high-quality public platforms are costly.
As a result, most small archives have few or no materials digitized and available online. Even large repositories like the Library of Congress do not have long-term plans to digitize more than a small fraction of their collections. Even if infinite resources were available, the fact is that most archives have a high volume of material that is likely to be of interest to only a very small population, so it is more efficient to provide access as the need arises.
Never interpret a lack of digitized material to mean that there aren't relevant archival sources for you. It just means that you may have to look at some finding aids or ask an archivist!