Hey Maureen, If you are searching for iron in the toner cartridges I suspect you are doing laser printed ceramic decals. This question often comes up and the best resource I have found is in Linda Arbuckle's tutorial on "Laser Decals for Ceramics." In her PDF, found here, she states:
"...you'll need to get special high-iron toner cartridges for your printer made for printing checks. Because the check numbers are read magnetically, the toner does still contain the high iron needed for a decal you fire. One company that produces this toner is Ganson Engineering Inc. www.ganson-store.com"
Maybe this ink made for magnetic checks will fit with the iron content you are looking for!
Best,
Justin Crowe