I could not find good copperas as my garden supply stores, the copperas came in bags and was always oxidized brown (which would not work to test for, or to precipitate gold with).
Copperas is easy for me to make, and when I do I make a big batch to last me for a while.
The iron core of the transformers is a good source of iron (not steel although, steel does have iron in it I would not use it). Pure soft iron is rare, steel is very common, steel can be a mixture of metals, it may work I do not know, transformers are also a good source for copper for your scrap metal bin, the Iron laminates are usually welded, you can grind off the welds with an angle grinder or similar tool and separate the laminates, they usually have a light coating of shellac so, heat them with a torch to burn this off, I cut mine up for faster reaction in the acid, I wash them with water just prior to using (so they do not rust), electric motor's also has an iron core's (not the steel shell case of the motor, these can be used but are a little more trouble to separate, I use a chop saw to cut these.
Copperas is also called Iron II sulfate, green vitriol, ferrous sulfate, or FeSO4.
Dilute sulfuric acid is used, about 10% acid, so if your using new battery acid (about 32%), add about twice your acids volume with water, it is not terribly critical but shoot for 10% acid, heating speeds the reaction, so I heat these add as much iron to your acid as you like, and make as much as copperas you want, the excess un-dissolved iron can be removed later, heat also saturates the acid with the metal, when solution begins to concentrate a nice green color I filter solution, into a jar, the heating vessel is cleaned out and the solution returned to the hotplate to concentrate to crystals, the solution can be used to precipitate gold But I make the crystals, as they begin to crystallize a fair amount while evaporating the solution I remove them from my vessel, and return the liquid to the evaporating pot, these light green crystals I store slightly damp, storing them acidic also helps so a couple of drops of sulfuric acid is added to the plastic storage jar, with a good tight sealing lid these will stay fresh until you wish to use them.
Fe + H2SO4 --> FeSO4 + H2
Beside precipitating gold, copperas crystals are also good to test for gold in a spot plate, (the brown gold precipitant as an indicator of a positive for gold in solution, they are also good to use in a test for PGMs when gold in solution may change the colors, here gold can be precipitated out in the spot plate on a crystal of copperas, the remaining liquid moved to another spot and retested for PGM, (this is a trick Harold has shared and he explains it better than I can.
I also use the bisulfate salts (the byproduct from making nitric acid), the bisulfate salts react in solutions like sulfuric acid, so we can used the acid potassium bisulfate with water and iron to make copperas crystals.
KHSO4 + Fe + H2O --> FeSO4 + K2SO4 + H2O + H+(g)
http://en.wikipedia.org/wiki/Iron(II)_sulfate
