Lead is a naturally occurring metal that can make its way through soil and water and into the foods we eat. For this reason, in the process of extracting calcium carbonate to make calcium supplements from oyster shells, some lead is present. In order to be compliant with federal standards, however, natural health products must not contain any more lead than an establsihed safe threshold. Most reputable calcium products (look for a natural product number on the label), and especially refined calcium products, contain either undetectable amounts of lead or amounts deemed to be safe.
In fact, these products often contain less lead than is found in green vegetables, and the calcium in the supplement typically prevents the digestive tract from absorbing much of those trace amounts of lead in the first place.
The one indisputable fact is that the safest and probably the most effective source of calcium for strong bones and overall health is diet, not supplements.