Have you ever heard of the Newberry Library? Founded in 1887 in Chicago by Walter L. Newberry, the Newberry has a vast collection of books, manuscripts, maps and other materials, many of which are available online.
The Library also has a genealogy blog that provides useful updates about holdings of interest to researchers.
Last month the Newberry announced it will allow open access to it's digital collections, no longer requiring licencing or re-use permission. That means when you find an awesome map showing detailed migration, you can not only download for your own use, you may use it in your family history book or blog without going through a lengthy process.
I recommend you check out the Newberry. Look at their online catalog. It is a great resource for your research.