They weren't super expensive to buy off ebay, probably a bit more expensive than the San-ei plushies you find off Amazon, but you can't find them easily in stores; Amazon will not stock these guys. Like, not even otaku or specialty stores will stock these guys because they're old. They were made all the way back in the 2000's and were Japan only in UFO catchers iirc, and while were probably common there, they were inaccessible to anyone outside of Japan. I heard about these plushies since, geez, maybe around 2009-2010 when I desperately wanted a Baby Luigi plush but I could have never got him.
So cut to today, where I finally own this variant of a Baby Luigi plushie including the diaper one and the little ones after all this time and I'm so thrilled for that.