My local library had a paperback swap rack, where people could take or leave books for free. One night, I went in and found several of the Ross DS novels, so I grabbed them and, on a hunch, asked the librarian if they happened to have any others that they hadn't put out yet. She went into an office and came out with a bag containing nearly all of the titles and handed it to me.
Another day, I was looking in a thrift shop at some books and saw, much to my surprise and pleasure, a copy of the HODS novelization, being sold for ten cents.
I eventually completed my collection through eBay, which at times was a bit pricey, but offset by getting the majority of the books for a dime.