Many libraries & archives have digitized collections of materials related to borderlands history and culture:
While not a primary source itself, Indigenous Mexico is a website that has articles based on primary sources that are linked within the article or at the bottom of the webpage.
19th Century Pamphlets
Tisch has a collection of mostly 19th century pamphlets in Special Collections that include topics covering the borderlands, including questions about annexation of Texas & California, debates and speeches on slavery and abolition, U.S. military campaigns in Mexico. To see a list of titles of pamphlets, click "view full text" under Available Online.
Rare books
Tisch has some rare books that cover the borderlands region, mostly travelogues and histories. To locate books in special collections, used the Advanced Search, with any field is exact "Tufts special collections" and your keyword (e.g. Mexico, Texas, California).
A few examples are below:
Contemporary Artist Books from the SMFA Collections
For more contemporary and artistic responses to the borderlands, students can visit the SMFA Library's special collections. A few sample titles are listed below: