Iron (II) sulfate is also known as vitriol, melanterite, salmortis, green copperas, or victriall. The “blood of copper” is derived from dissolving nails or other iron scraps in acid and then using the metal salt left remaining after evaporating the solution—sometimes with the addition of alcohol.[1]
Green copperas is also found around natural springs near iron mines when acidic runoff reacts with iron ore. Realistically, most people in need of green copperas would purchase it from their local alchemist, later their pharmacist, and now from chemical and mining supply companies.
Copper sulfate (blue vitriol, blue copperas) was sometimes substituted in place of the iron (ii) sulfate; this would give a more brown hue to the resulting ink.
[1] http://web.ceu.hu/medstud/manual/MMM/ink.html