Walt was cremated and his ashes placed at Forrest Lawn Glendale. You can find his grave in a small garden outside The Freedom Mausoleum. I was able to visit it a few months ago during a trip to California.
Here is the entrance to Walt's Garden.
And a shot from the inside.
Buried with Walt are his wife Lillian, and their son in law.
If you are interested in paying your respects to Walt you can visit Forrest Lawn Glendale. The employees are not permitted to tell you where someone is buried though. If your looking for Walt follow the signs to The Freedom Mausoleum. His garden is on the left hand side of the entrance. The first page of my Flickr has an aerial shot with his garden circled.