Looks like the shooter got his rifle in the Binghamton area (where my dad lives), specifically in Endicott. It was a Bushmaster XM-15 that had been modified to comply with the NY SAFE Act, including to have a fixed internal 10 round magazine (the shooter called this the 'cucked' version), which he then illegally modified to take higher cap swapable mags.
Trying to verify now, but it looks like he had an 180 page plan and explanation prepared.
EDIT: Source was his alleged Twitch account where he was livestreaming the attack, now taken down. Could be a fake, but found very shortly after the shooting by people claiming to have been on the start of his stream, and with details not publicly released until a couple hours ago.
EDIT 2: Police have confirmed they are investigating the 106 page manifesto (the .pdf is 180 pages, but only 106 have material). The Twitch account that the online sluths say it came from has been verified to have been the shooters.
EDIT 3: And unsurprisingly the manifesto advances the same 'Great Replacement Theory' pushed by the likes of Tucker Carlson and Trump, which lead to the shootings at the Tree of Life Synagogue, the El Paso Wal-Mart, and two mosques in New Zealand.