Online platforms like Amazon or Barnes & Noble's website are good places to start. They have a wide range of books, including light novels. You can also check out some specialized Japanese book - selling websites if it's a Japanese light novel as they might have a more comprehensive collection.
Another option is to visit libraries. Some libraries have a growing collection of light novels these days. You can search their catalog either online or in - person to see if they have the 'index light novel' you're interested in. Additionally, there are some fan - run communities or forums where people might discuss where to find rare or hard - to - get light novels, and you could ask there as well.