Online and in librarys and museums. Bankruptcy and estate sales can be good places to find data. It is important to pay attention to data licensing. Personal reference data is good but it is even better if find public domain data or information that can be placed into the commons under a GPL, FDL, and/or creative commons for everyone else to use as well.
I have been told that NASA has the engineering drawings for the Saturn stack somewhere in their data warehouses. I doubt they have the rights to the manufacturing specifications for the various components that contractors and vertical chains of subcontractors used in helping manufacture the moon mission launch stacks and space vehicles.
To be able to reliably maintain our equipment in remote hostile environments we need manufacturing specifications for parts, manufacturing processes, and factory's that we can emulate on short notice if necessary.