The Northwest Digital Archives (NWDA) gives you information on archival collections throughout the Northwest. Most of the collections are not digital themselves, but you can use this archive as a finding aid--to figure out where you might want to visit to find materials relevant to your topic.
Using a special collections library or archive is different from using public and academic libraries. For example, not all of the items in the collection may be cataloged, and you will probably not be able to browse the collection. The organization may have limited hours. There may rules around what you can bring into the reading area: some will allow only pencils and not pens, and you should plan to leave your food and drinks at home.
To prepare for your visit, explore the organization's website to see what collections they hold. Check the organization's hours. And then contact the organization to make an appointment, as not all archives are open without appointment.