I agree she is another of those weird ones who has suffered due to being in the second wave of a set (which means they will be traditionally lighter in numbers anyway) and then having missed out on the US altogether due to timing and probably over stocking of the earlier set.
She was easy enough to come across here for a while but the fact she was again in a wave with main 6 ponies meant she was the only "unique" character on shelves. So no doubt often the first one picked up. She was available on The Entertainers website for quite a while even after we all clocked she was not going to turn up in the US. So I would guess from the amount of time she was available that a lot of people were able to track her down. But we have since moved on to the next line here unfortunately.
She may very well get a run out again in the future - we have seen a few characters getting repeat releases after all. But in case she doesnt I would agree with watching for a factory reject on ebay - they turn up reasonable often