You can get them directly from Weber but you probably want to do that when they offer free shipping. Unless you need one which is no longer available, Weber or one of the other vendors listed above would be the easiest.
If you ever get over to the south Chicago 'burbs, there is a place in Bolingbrook (IIRC) that sells 9 Ga expanded metal for $35 for a 4x9' sheet. That's a lot heavier than the stuff you find at Home Depot. (Can't find a listing right now though.)