British Parliament passed an act in 1750 which was intended to encourage the production of pig iron and bar iron by the colonies and to prohibit the construction of any
more rilling, slitting, or plating mills. The idea behind the Iron Act of 1750 which dropped import duties of iron going to England, was to make the colonies increase production and export of the two types of raw material iron (bar and pig) but to limit any additional development of the more developed technologies for making nail rod and sheet iron. There was no attempt to close existing rolling and plating mills.
A quick Google search will reveal a lot of the details including why the act was unsuccessful, why it is considered one of the causes of the Revolution and that it was obsolete but not repealed until the mid-19th century.
The bottom line, in terms of this discussion, is that sheet iron was being made in the colonies prior to 1750 and that production continued in direct competition with imported sheet. Sheet iron was likely available anywhere sheet brass was available. This leaves the real question, as Wallace Gusler often points out, "Why weren't there more iron mounted rifles, especially in areas with German influence?"
Below is a link to one such on-line resource. There are dozens.
Gary
http://www.answers.com/topic/iron-act