Where can I find information about the nickel content in foods?

The FDA's Total Diet Study results include nickel content of a nationwide sampling of about 300 food items. FDA provides tab-delimited summary files on its website.

In addition, the Mayo Clinic has information on Nickel Allergy, which briefly mentions diet under the Causes section. Diet is also mentioned toward the bottom of the NSC's Nickel Allergy, from the government of Singapore.

You may also try searching PubMed for research on nickel allergy and the nickel content of foods.