Here are the only links I have found for John Gardner and his works:

John Gardner general:

• JOHN C. GARDNER APPRECIATION PAGE
• The Arch and the Abyss: A John Gardner Resource
• Primary Sources Includes first edition information for all of what might be considered "significant" publications by John C. Gardner.

Essays and such about Grendel

• A list of banned books that Grendel is part of
• A permission letter to read it
• "Grendel," by Stacy Kluck of North High School, Eau Claire, Wisconsin.
• "Grendel" by Roxanne Fuentes, 1996, AP English IV, Loretto Academy.

Places where you can purchase first editions of Grendel and other books.*

*Please note that I am not advertising for any of these buisneses. I personally think you should give your buisness to a local dealer, but for the sake of convienence I have included these.

Just a big bookstore

Between the Covers, rare books

Last I checked this had a copy of Lord of the Flies, another of my favorites, previously owned (signed) by John Gardner, and a heavily annotated work of Poe, owned and signed by John Gardner.

Advanced Book Exchange:

Here you can do a search for any rare book. First editions of Grendel run for about $125-200 American.