Hi James,
Don't know what your circumstances are for obtaining any locally (radio shack, Comp USA, Office Depot, Staples, etc) since many retailer just end up throwing them away after installing the components they were in.
If you are unable to find any anti-static bags large enough but have several medium size ones you can tape them together (on the outside with 2 inch tape) to give you the size you need. In a pinch I have had to do that several times for very large boards I have and seems to work out ok.
Richard