Where do I find the item/source record to load?

I suggest you look at catalogs from your library, the Library of Congress, WorldCat, etc.  I love the catalogers at Cornell and New York City Public Library  and Harvard for their historic notes.  Once you find the good record capture/add it.  Remember if the source changed its title or formats or publisher please capture those records too.  GPO and Alma catalogs do NOT work well with Zotero.  If you do create a unique record remember to put your name in the Library Catalog line.  For the most current source, might only be found by searching the web (not a library catalog) by the last know title of the source.  Option: I capture all the records that I can find and create one good record.