I looked into it at one time and was able to find a company that provides all types of IP fixtures. I can't remember who it was now. The reason I didn't keep the information was, it was cheaper to make my own. The price they charge for the fixture is more than rediculous. It is actually cheaper to take the specifications to a machine shop and have them turn out a couple for you.
I have worked for several agencies and most just use a very fine mist shower head like the ones they use in the Gym locker room. It's more important that you make sure the angle and all directions are applied and are correct for the different IP water ingress test. If anything you are performing worse case testing using a standard shower head.
The harder test is the dust test, it requires you suspend Talc in air in a chamber. I was able to simulate this as well. Read the spec closely and see if you are able to simulate the same environment. Thats all you really need to do.
