I'm not certain. You could try looking in local libraries, especially in the children's or fantasy sections. They might have a copy of the book if it's a printed story.
You could also check with independent bookstores. They often carry unique and hard - to - find stories. Another option is to ask around in reading groups or forums. There might be someone who has heard of or read the 'Sea Monkies Great Big Story' and can give you more information on where to find it.