How do I find my great grandfathers adoption records?

Lots of luck.

Seriously.

You cannot have those records opened if they are closed.

What year are we talking about here? Prior to about 1910, adoption records were not even KEPT. The adoptive parents were just put down on the birth certificate of an infant.

What do you want the information for?

Rosey,
Are you seeking medical history information for your grandfather? If you know a name, or the adoption agency used, please start there. And join an adoption search group for answers. There are many search angels equipped to help in the individual state that you are searching in for example. Here is a link that may or maynot get you in the right direction:
ABC's Favorite Links (http://www.adopteesbirthrightscommittee.com/links.html)