How to find old editions online, though? Unless you want to buy a very expensive rare edition, IMSLP (The Petrucci Music Library) may be your best port of call.
IMSLP and Petrucci are the same website. They describe themselves this way:- "IMSLP stands for International Music Score Library Project. The logo is a capital letter A, taken from the very first press-printed book of polyphonic music, the Harmonice Musices Odhecaton, published in 1501. Its printer, Ottaviano Petrucci, is this library's namesake."
Here are a few examples of what you'll find:-