Your options, as we see them:
-shop online (aka The Expensive, Easy Way)
-hunt yard sales (aka The Really Cheap, Difficult Way)
-hunt independent bookstores (aka The Kinda' Cheap [if the booksellers don't realize they're sitting on a gold mine by anifan standards] Way)
-library sales (aka The Way that Involves Waiting A Really Long Time for the Books to Hit the Discard Shelf)
-beg somebody else out of their books (aka The Incredibly Unlikely Way)
Of the 48-and-some-duplicates collection I have at home, about half were found at a single yard sale. Another six were gifts, twelve were bought at book fairs and chain bookstores when the books were still in print. One was found at a library discard sale. The rest were found at various independent booksellers in Illinois and Wisconsin.
As it stands... unless the series goes through a full reprint, owning a complete set is dependent on your wealth, luck, and/or persistence.
Regardless, good luck building a set!