If you want quick access and solid context, I head to a mix of primary-source aggregators and curated museum exhibits online. Project Gutenberg and Internet Archive are my first stops for full-
text books and scanned journals—those sites host everything from
guidebooks written for hopeful forty-niners to firsthand diary compilations. For newspapers, Chronicling America (Library of Congress) is gold: you can search by date, town, and keyword to find contemporaneous reporting and public notices.
For curated selections, Calisphere and university digital repositories (Bancroft Library, UCLA, and others) are excellent because they often provide transcriptions, maps, and explanatory essays. HathiTrust and Google Books fill in items that are out of print but digitally preserved. If you’re doing research beyond public domain material, JSTOR and
university library portals often have scholarly articles and edited collections that explain biases and context—some are free, some behind paywalls. I usually mix firsthand diaries with modern commentary so the sources and the scholarship talk to each other, and that mix keeps the story alive for me.