OK! Where is your linkbutton? In EditItemTemplate or FooterTemplate? Then do this way
Control control = null;
if (GridView1.FooterRow != null)
{
control = GridView1.FooterRow;
}
else
{
control = GridView1.Controls[0].Controls[0];
}
Linkbutton lb = (control.FindControl("Linkbutton1") as Linkbutton);