You can try local libraries. Many libraries have a section dedicated to children's books from the 90s, including science fiction ones. They might be in the general children's area or in a special collection for older children's books.
Second - hand bookstores are also a great option. These stores often have a wide variety of older books. You could look for ones that specialize in children's literature or science fiction. The staff there might be able to help you if you ask about 90s science fiction children's books.
Online platforms like Amazon or eBay can be useful. There are many sellers on these platforms who offer second - hand books. Just search for '90s science fiction children's books' and you'll get a list of available options. However, make sure to check the condition of the book and the seller's reputation before buying.