[skissane]

The easiest legal way to get an AIX box is via IBM Cloud. You can get a tiny AIX box (s922 in Dallas, 0.25 cores, 2GB RAM, 10GB disk) for about US$50 a month. About US$8/month of that is AIX licensing and the rest is the hardware cost. Still a lot cheaper than IBM i licensing, which is US$350/month even for such a low-end system, or IBM Z (which is around US$1600/month for a low-end system).

IBM Cloud does offer some free credits – at the moment US$1000 free VPC credits – but I'm not sure if you can use them on AIX (which isn't part of their main VPC offering, POWER is a separate offering), and even if you can, they aren't going to last forever.

If one is an open source developer doing this on one's own time, as opposed to a business – why spend US$50/month to get an AIX environment? Maybe, if a user of the project really wants AIX support, they could donate that to the project.