It might be worth reading Wickard v. Filburn (1942)[1] for the original case that intrastate commerce affecting interstate commerce and therefore regulatable by the federal government. In US v. Lopez (1995)[2], the Court does actual say there are limits to what the federal government can lump under "interstate commerce," as they declare that the Gun-Free School Zones Act was beyond the power of Congress.
[1] https://en.wikipedia.org/wiki/Wickard_v._Filburn
[2] https://en.wikipedia.org/wiki/United_States_v._Lopez