As a libertarian, I always fail to understand where the doubt on self defense comes from, exactly as with the question on "who will build the roads", for which the answer is the same in this case: defense is much a service as civil engineer, and since even right now all defense providers are from the private sector, in a libertarian world defense would be administered by subscription just like any other current defense system in the private sector: you build your defense proposal, and cover the ones who pay you. You will then say "but that could lead to have different defense partitions that could be hostile to each other precluding coordinated effort", and I would say "tell that to anyone in service and he will not be able to tell the difference with current status of matters".