If you think about it, these are the distant ancestors of todays High Grade figure collections. Plastic Pachi Kaiju lasted through the 70s, only to be replaced by gomu (ゴム) or rubber figures. In the late eighties to early nineties, gomu figures really advanced in detail and were replaced by the HG, while gomu would be used primarily for super deformed figures.
But I digress...I look for every way possible to add to my Pachi Kaiju collection. Usually, the toys themselves are pretty cheap, since they don't have a copyright mark on them. The trick is to have someone see enough similarity with what they are supposed to be, to either end up on Yahoo Japan or at the stores.