MicroPython is developed using git for source code management, and the master repository can be found on GitHub at github.com/micropython/micropython.
The full source-code distribution of the latest version is available for download here:
Daily snapshots of the GitHub repository (not including submodules) are available from this server:
Firmware for various microcontroller ports and boards are built automatically on a daily basis and can be found in the pictured sections below. Alternatively, a list of all available firmware is here.