Make the rounds at yard sales and flea markets. Flea markets often feature sellers who have boxes of romance books, with prices ranging from 25 cents a book to typically no more than $2. Since these dealers acquire used romance novels in bulk at little cost, you can often ask for a deal if you purchase from them in quantity.
Join PaperbackSwap.com. This is a way to trade the books you have read for credits. These credits allow you to purchase used books. When a member of PaperbackSwap requests a book from you and you mail it to them, you are rewarded with a credit you can use toward a book. There is no charge to join PaperbackSwap.com and you can quickly build a library of used romance books from their huge selection.
Shop eBay, Amazon and other online sources. A quick "romance" search in the books category will bring up many results. Amazon features new and used selections, so be sure to click the "used" option to find bargains. Vendors at eBay often offer romance books in lots that include complete romance series books, so look for "lot" deals.
Shop at the many used romance bookstores online to find deals on used romance book (see resources). Check the shipping cost when you purchase online to be sure you're getting a good price on used books.