Which source has the best credibility for a research paper about the Battle of Gettysburg?

A main source. Mostly an old book that thoroughly describes the battle. First hand accounts also work, ad there are many of therm. General Lee's journal is a good source, where he talks about the battle.

A secondary source could be an encyclopedia or a third-party book/source where the author talks about the Battle and what happened during those three days ...

A good search on the internet will yield you good results.

TCI learning is a great website