They're very common overseas. When I lived in Shanghai, the markets would have tons of these. Unfortunately, they aren't always ludicrously oversized/underpriced. I had some data I lost on a 16GB drive that was only 4GB(IIRC). Granted, that was years ago
You can see them on Amazon if you look for flash drives. A few manufacturers will have really large drives (256GB or more) for ridiculously low prices, along with a 1-star rating and a lot of people claiming they are fakes.
There are plenty of legit companies selling devices that are legitimately those sizes, but you will pay a lot for them.
Of course, since flash media prices seem to drop by 25% or more every year, it won't be long before there are real devices at the prices of those fake ones mentioned above.
