Hmmmmm.
I've never even thought of eBay; that might be worth looking into.
Garage and estate sales, and antique dealers, I've tried before, but essentially, with so few people in this area, even for 200 miles around, we don't have many of these sorts of places.
As mentioned, Replacements Ltd. is good, great, but there's some stuff that's just too damned hard to find.
When I've added to--we're talking 7 different sets of antique china here--wanting, for example, two cups, but could get them only by purchasing other parts of the set, I've had no problem doing that. I just figure that over time, a set evolves from an 8-place setting to a 10-place setting, to a 12-place setting, and so on, and so it's no big deal. If I have to buy other pieces just to get a cup, it's no big deal.