I am looking for a hole cover for water/sever line outside in my front yard. It is 9 inch in diameter. I could not find it in my local home depot. Can you tell me where can I find it?

I'd look for plumbing supply houses near you. They will have and can get many things you won't find at Lowe's or Home Depot because they deal with people that work in the plumbing trade. You can find them in your local Yellow Pages.

Those are usually installed by the local water and sewer utility when the houses are built Check with them.