Since this is a relatively short work, look for it in collections of "Old Russian Literature" (Древнерусская литература) on digital libraries like Liters or community archives; these collections almost always include the tale in FB2 format. Quick Summary
You can access high-quality digitized versions and transcripts through the Pushkin House Electronic Library or the Russian National Electronic Library (Rusneb) .
For academic study, Dokumen.pub hosts the 1914 edition by B.I. Dunaev, which includes the original Old Russian text.