In addition to Wellesley's rich collections, the Boston area has many libraries and archives that are accessible to local researchers. Check their catalogs to locate potential items of interest and check the institutions' access policies prior to visiting.
The following nearby libraries are part of the BLC.
There are many libraries and archives who scan or photograph their documents and other sources and put them up online for you to use.
Here is a sampling of places you can go to find primary sources, depending on your time period and topic.
This doesn't represent everything out there for you, however. If you'd like to brainstorm more sources and places to look together, make an appointment with me!