Works in the public domain -- pretty much anything pre-1920s -- are usually easy to find online.
Project Gutenberg is a good place to start at https://www.gutenberg.org.
Public libraries are great resources. Search the San Francisco Public Library at https://sfpl.org.
The Alternative Canon is a joint effort to list and provide links to non-Western plays, and plays by BIPOC, women, and queer writers before 1945 and is available here -- https://docs.google.com/document/d/1HwAHAwU83bW_jSzxbMrqWyyuAYmsOWtqK-7HxKA9yek/mobilebasic
Expand The Canon has a list of female classic playwrights at https://www.hedgepigensemble.org/expand-the-canon
A bunch of the Greek plays are available here -- https://theatrelinks.com/greek-playscripts/
Fragments from lost Greek plays are here -- https://lostgreekplays.com
There are a lot of great Shakespeare sites and sources out there.
Shakespeare scanned -- http://prescannedshakespeare.aruffo.com
Complete works -- http://shakespeare.mit.edu