Well, thrift stores could be a great place. They receive all kinds of books, including light novels, and might sell them at half price or even less. Also, some libraries hold book sales periodically. At these sales, you can find light novels at really good prices. It's like a treasure hunt. You might have to dig through a lot of books, but it can be really rewarding when you find that half - price light novel you've been looking for.
You can try looking in local second - hand bookstores. They often have a section dedicated to light novels and might offer them at half price, especially if they are older editions or have been in stock for a while. Another option is to check online marketplaces like eBay or AbeBooks. Sometimes, individual sellers list light novels at discounted prices there.