I don't know, but I've been using my BECO waist extender on my new Ergo and it fits perfectly. I bought the extender for the BECO, but I don't use it on the BECO becasue the straps are long enough. But the ERGO was tight so I tried the BECO extender and it fit fine.
Ok...nevermind. I just realized you are asking about the old style ERGO. Sorry!
If you don't find one, just make one. In the end, it's just some buckles and nylon. You should be able to find this at a local outdoor store or online. Simply match the buckle width and buy about a foot of nylon webbing of the correct width to thread in the buckles. Then have a seamstress do an industrial job sewing the nylon webbing. It's got to be done the way the existing ones are.