If you want to purchase the Japanese version of a light novel volume, manga volume, or anime disc that was released relatively recently, Amazon Japan is almost sure to have what you're looking for.
Since you know at least some Japanese, you can just search the site for it. Unless you want to use a proxy-shipping service, you will want to find products that ship internationally, which you can do by filtering your search down to "AmazonGlobal対象" products. Upon doing so, you will find that a print version is available here, and also that a Kindle version is available here, which may be cheaper overall. (But note that I've had issues with Kindle purchases from Amazon Japan - the various country-specific versions of Amazon don't share information about your Kindle purchases with each other, which seems to cause problems with some Kindle devices/apps.)
Depending on what country you live in, there may be other options that are cheaper, more convenient, or both. For example, if you reside in the US, Kinokuniya USA will ship basically any anime-related thing to you (more convenient), but charges a fairly hefty markup for it (not cheaper).
Durarara!! and most other light novels are generally unlikely to be found for free from any legitimate vendor, though occasionally the publishers will upload the first chapter or two online for free as a sampler.