Resources
Commercial Pattern Archive (CoPA) at the University of Rhode Island
While the sign-up process is a bit clumsy, this website is well worth the effort. The database indexes thousands of sewing patterns and usually includes scans of the front and back of the pattern envelope. They have a robust search feature allowing you to search and filter by year, brand, pattern number, style, etc.
The Midvale Cottage
While this blog has been inactive since 2017, their posts on vintage pattern sizing (what ARE half-sizes anyway???) from the 1920s-1960s are incredibly helpful and full of wonderful period illustrations.
1920s and 1930s Pattern Sizing
How to Resize a Pattern from Sensibility Patterns
This is an extremely in-depth blog post on pattern resizing.