While the code for the web launcher (the "wrapper" that runs the game) is often open-source on GitHub, the assets themselves are proprietary. The community generally respects the developers by acknowledging that they should purchase the official game to support its continued development. Eaglercraft is best viewed as a technical novelty and a proof-of-concept rather than a replacement for the official client.
If you are looking for a server based on (often referred to as "Deep" or Universal Paper servers), these repositories provide the necessary jars and scripts: eaglercraft 112 github link