Here is a link to the present discussion of the Mercury MACT. http://epa.gov/ttn/atw/hgcellcl/fr11jn08
MACTs are listed as NESHAPS or NSPS documents. It will read something like: NESHAP MACT C.F.R 40 subpart [Mercury Mact].That will help in any further searches.
Under your State Department of Environmental Quality, State Environmental Protection Agency, of Department of Pollution & Control (a couple other kind of names like that).
This particular MACT is usually over-seen by the Haz Waste division. Most NESHAP's are a combination of Haz Waste & Air, while NSPS documents are Air & Water.