I don't like using Wikipedia for a source, but here's some paleoclimate data going back to the early Phanerozoic.
https://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/Al...
Also, I /highly/ recommend reading over Hansen et al's 2013 paper
It's quite lengthy but it should answer a lot of questions you may have:
https://journals.plos.org/plosone/article?id=10.1371/journal...
Oxygen isotope ratios are more or less ^16O and ^18O. These isotopes are distributed differently based on temperature and precipitation patterns, which can be recorded in ice cores, ocean sediments, lake sediments, and even in the shells of marine organisms. ^16O is more often seen in 'warmer' events, vs ^18O
NOAA's GHCN: Global Historical Climatology Network, Its coverage includes data going back several centuries, with the earliest records starting around 1750.
HadCRUIT (Hadley Centre/Climatic Research Unit Temperature). Has marine (ship-borne and buoy) and terrestrial (land-based stations) sources.
NASA's GISS Surface Temperature Analysis has records from the 1880s.
As for calibration, I believe some places routinely calibrated their mercury tstats. But since that was a potential issue, we started using indirect measurements (eg: what I mentioned) to confirm certain climate trends.