FYI: You cannot purchase health insurance across state lines ...
"Comprehensive as the word "among" is, it may very properly be restricted to that commerce which concerns more States than one. The phrase is not one which would probably have been selected to indicate the completely interior traffic of a State, because it is not an apt phrase for that purpose, and the enumeration of the particular classes of commerce to which the power was to be extended would not have been made had the intention [p195] been to extend the power to every description. The enumeration presupposes something not enumerated, and that something, if we regard the language or the subject of the sentence, must be the exclusively internal commerce of a State. The genius and character of the whole government seem to be that its action is to be applied to all the external concerns of the nation, and to those internal concerns which affect the States generally, but not to those which are completely within a particular State, which do not affect other States, and with which it is not necessary to interfere for the purpose of executing some of the general powers of the government. The completely internal commerce of a State, then, may be considered as reserved for the State itself."
Of course Wickard v Fillburn changed all that. Raich recently reaffirmed Wickard (with Scalia concurring.)