Assuming you're using Entity Framework, you can pass the search into the database so that you're not loading more records than necessary.
DAL:
public static List<Piskaot> GetSrcPiska(string src)
{
using (var context = new DBNafshenuEntities())
{
return context.Piskaot.Where(item => item.Tohen.Contains(src)).ToList();
}
}