NiDB is available as open-source software or as a commercially hosted instance on Amazon Web Services
Download the rpm and install on your own server. This is a great option if you have experience installing and maintaining Linux based software.
Available from https://github.com/gbook/nidb
This is a better option if you have no experience managing a Linux server. Maintenance, backups, upgrades, and support are part of your contract.
Visit NiDB on AWS Marketplace