Finding 'Pelari Muda' online can be a bit of a puzzle because its availability isn't uniform across platforms. I looked into it after seeing a recommendation on a local bookstagram account. You're most likely to have success with Indonesian-focused retailers or digital libraries. I got my digital copy from Gramedia Digital, which is pretty straightforward if you have an account there. Google Play Books sometimes carries it too, but stock seems to come and go.
A friend mentioned she found a physical copy on Tokopedia through a seller specializing in local literature, though that was a few months back. For an ebook, the Gramedia app was my route, and it synced fine to my tablet. Just check a couple of those spots; one usually pans out.