You'll be lucky to find the books Steve, I suspect. Moomins are out of fashion, and libraries in experience are ruthless these days. They have a Top Ten attitude, especially towards children's books.
Still at least most of the Moomin books are still in print. Unlike the Uncle books by JP Martin which I'd recommend to anyone who likes Moomins (different in tone, more rumbustious, but there's a similarity as well) - though some of these are now back in print. (And here is a link to a page about Uncle.) And here is a page with a chapter from one of the Uncle books.
The essential thing about greatest children's books, I believe is that they are not condescending. There's never a sense "I'd better keep it simple for the kids. Or "I'll stick this to keep the little devils happy." Even with the Harry Potter books, which I think are pretty good, there sometimes seems to me to be an element of that latter attitude.
But never with Tove Jansson. (Or JP Martin.)