some items such as boron, magnesium, and sulfur are found in common grocery superstore place as borax and epsom salts.
others like iron chealate and, I believe, zinc sulfate are bought at farm supply stores (i.e. southernstates)
I use a 5-11-26 soluble hydroponic solution from southerag.com that already contains all the trace elements.
I bought it through a local landscape supplier who buys from then regularly. I believe it was about $35-$40 for a 25# bag.
When I learned about it, they would ship directly and let me tell you, 25# will last you a long time.
feel free to contact me if you need some assistance locating it.

Any chemical supplier can sell you the salts that contain all the necessary micro nutrients. For example, you may purchase

Magnesium Sulphate, Boric acid, copper sulphate, manganese sulphate, zing sulphate, iron chelate (DPTA) and sodium molibdate

in order to provide all the necessary nutrients to your hydroponic solution.

Here is a link that might be useful: Everything Hydroponics