Getting a Banalinga (also known as Narmada Shiva Linga or Narmadeswar Shiva linga) is quite easy these days.
You will get them in shops selling religious items. And, they are also sold on websites like ebay.
My question is how to determine whether the stone is really Lord Shiva or just any other lifeless stone?
Getting a Banalinga is important in the sense that since the Lord is already said to be present in it the elaborate procedures of pranapratistha are not needed to invoke the divine presence.