There are a variety of sources. Think about looking at some sources on the World Wide Web, then see whether your local hardware stores carry any similar material.
Some existing vendors or sources for netting are listed below.
Bird Barrier Affiliates
Wildlife Control Store
Canadian Vendor List
Fish and Wildlife Service Vendor List
Just remember to look for netting that has been UV stabilized. Netting breaks down under sunlight. Purchase netting that is guaranteed for at least five years.
You can also consider using 1/4-inch hardware cloth to exclude smaller openings.