I am trying to find the source that says that a zar who shechts a korban needs to stand outside of the azara and use an extra long knife to perform the shecihta (since a zar is not allowed to go into the azara). I am having trouble finding the source. Can you please help. Thank you.

ב"ה Shalom It seems the source you're referring to is the Gemara in Zevachim 32a-32b. The Gemara is talking about a hypothetical situation in which someone is "Tamey"=impure and is standing outside the Azara with a long knife and is able to do the shechita. All the best