You really need to explain what you mean by "Unix-like" and by "pure".
So by "simple" you mean short source code? Minix might be good.
Do you mean the original written by Dennis and Ken? I don't think that source code is available (without a license, and I have no idea where one would get such a license). And even if you managed to get the source, to run it you would probably first have to buy a used PDP, or find a PDP emulator. And the result would be surprisingly boring compared to a modern system.
If you mean "new development, more feature-rich, but done in the philosophy of the original Unix", then it really depends on what you define as the "philosophy" of Unix. Richard Stallman and Rob Pike don't agree on it.
If you want free software (in the sense of being able to inspect the source code), you need to think through why you want that, and what you will do with it. Have you tried reading the source code for a complete Linux distribution? Just reading the kernel source (and learning the programming skills required to understand it) would take you years. If you add the normal user land that is on a standard Fedora or Debian install disk, it would probably take a human centuries to read it. So again, why do you want the source code exactly?
And the idea of using AIX as a particularly Unix-like OS is funny. IBM did a lot of stuff to modify AIX. Here is an example:
Normal Unix: "ls foo" -> "foo: No such file or directory"
AIX: "ls foo" -> "The dataset foo can not be located in the current catalog."
And Plan 9 is nowhere Unix like. It is a vastly expanded descendent. I'm not saying that it is bad, only that it is very different.