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 (link is external) on its website.
In addition, the Mayo Clinic has information on Nickel Allergy (link is external), which briefly mentions diet under the Causes section. Diet is also mentioned toward the bottom of the NSC's Nickel Allergy (link is external), from the government of Singapore.
You may also try searching PubMed (link is external) for research on nickel allergy and the nickel content of foods.