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?
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.