There's an open-source version that's available:
https://github.com/kungfuant/ (look for 'bustabit-gameserver' and 'webserver') which you can use free of charge if you comply with the license.
Or if you want to pay, you can talk to
daniel@bustabit.com who sells licensed (bustabit.com/license.txt) versions which is a bit more updated than the opensource version and comes without restrictions.