In my experience they're not that rare, but it's finding them that's the tricky bit. The main problem is that scrappers or even collectors come across them and don't know what they are. Like many other such things, it's quite possible you could pick up a bargain if you are looking over a long period of time on eBay using certain search terms. What is more likely is that you will pay more because the item is listed specifically, because the seller knows what they've got (or you're impatient - sometimes you just need it fast).
Amibay might be a good place to look because you can get less common stuff for more reasonable prices.
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