You can find it in GitHub, licensed under the MIT license.
TameJs is actually based on a similar technology created by OK Cupid but designed for C++:
OkCupid serves externally over 100 million dynamic HTTP requests every day (over 1,000/second on average), each of which fires off calls to all kinds of other services, literally billions of async calls daily. Everything is Tamed, and we'll never look back.
The C++ version is also available, however "it requires committing to certain other libraries you might not want (sfslite, libasync)."