I phoned Breville in Sydney, *1300 139 798, asked for a
"ESP4/51 coffee filter 1-2 cup",
paid $9 each and $10 postage over the phone and they sent it up the next day. (actually I bought three, as the postage stayed at $10, as Xmas presents for my son, his parents in law who put me onto the 800, and for me).
The filter is great, it fits beautifully and has a tapered bottom, which holds slightly less grounds, and if you leave it attached to the group head for a minute or two after pulling the shots, the puck dries and *drops out nice and clean.