Instructions
Decide which set(s) you wish to collect, the original books consist of numbers 1-34. The same person did not write the books, but Grosset &Dunlap published all. There are other modern versions as well.
Make a list of the titles and copyright years you are looking for. Carry this list with you everywhere. Books published between 1930 and 1959 will have 1930 listed as the copyright date. First editions will also have 25 chapters (books 1-34).
Look closely at pictures of the covers of the titles you wish to collect. Each series looks different so make sure you know what you are looking for.
Search online auction houses such as eBay for books. Be cautious about who you buy from, make sure they are a reputable seller.
Search used bookstores, thrift stores, flea markets and book fairs.