A true free market?

Somalia is an example of warlordism, not anarchy. A powerful warlord coerces others and declares the territory under his influence as a quasi-state with the warlord himself being a quasi-king. Anarcho-capitalism, on the other hand, is based on the non-aggression axiom.

(Sorry to butt in, Neb)

Is there an example of anarchy that is not specially disqualified from being evidence against that particular system?

Anarchists, I feel, are the type of people who say that by definition anarchy must work. When it doesn't, it's not "true" anarchy. I've had to beat this problem out of a self-confessed anarchist before, he's almost about to grow out of it.

Depends on how you define "government." Somalia is stateless, but not "true" anarchy. However, there is similarly a big problem with describing anarcho-capitalism as anarchy. If you want, you can go back to pre-historic hunter-gatherer groups, but they weren't very capitalistic, plus they lived in (obviously) massively different cultural circumstances and a significantly different biosphere with much lower population levels.