As you've already done some research I can confirm that KubeMQ is a paid solution and it's not possible to use it for free for production usage (it's possible though to have for development purposes for half a year - license details can be found here)

There are good alternatives you can focus on:

• RabbitMQ - robust and very reliable solution (here's how to run it in kubernetes)
• Apache ActiveMQ

I think they have an open-source edition now with Apache 2.0 license

KubeMQ Community is the open-source version of KubeMQ, the Kubernetes native message broker.

as in this link link