I haven't seen anyone else cite it; it's from the Journal of the Virginia House of Burgesses, June 2, 1757, reporting on a "Petition of sundry Freeholders, Inhabitants of the County of Middlesex," which set forth (emphasis added):

That a Sum, not exceeding £1000, was expressly limitted by Law, for constructing a Fort at Winchester, and they are informed £10,000 hath been expended. — That a well regulated Militia is the true and natural Defence of every free State, and praying that the Expence of building the Fort, and the Conduct of the Forces in the Pay of this Colony may be enquired to: And that if it is necessary to lay any more Taxes the same may be laid on the nett Produce of the Planter's labor, and raised within the Year ....

Further evidence, it seems to me, that "free state" in the Second Amendment means "free country", and not "state of the union independent of undue federal power." In 1757, no-one was thinking of Virginia as a "State," or talking about its independence to the colonial legislature. But, as Blackstone made clear in 1765, people were reasoning — whether or not correctly, or in a way applicable to modern conditions — that a free (in the sense of nontyrannical) country was best defended by the militia. And if the Colony of Virginia was seen as part of the free State called Great Britain, then it's quite reasonable that D.C. would be part of the free State called the U.S.