A Google search under "Lime Sulphur" and then search within results on "Iowa" turned up this page:
http://www.linnextension.org/mg/public/austree.htm
It contains the phone number of Iowa state university extension service for Linn County (How close is that to you?)
While they are not talking bonsai, they do recommend the use of Lime Sulphur. Call and ask them where to get it in retail quantity (You really don't want to buy a barrel, do you?). If they say "any garden center", ask them for specific suggestions.
p.s. A lot more hits can be found if you spell it "sulfur"