Missouri Meerschaum in Washington, Mo., has sold replacement stems in the past, as have many retailers, but I haven't bought replacement stems in years. The standard black and amber-colored (not to be confused with the material called amber) are too soft for me, and I chew through them, so I have replaced my MM cob stems with Forever stems, and more recently, bought the special edition pipes and others that come with acrylic stems that are far more durable.
MM cobs last a long time for me. I don't consider them disposable or temporary. Especially if you have quite a few pipes, they can last for decades or a lifetime.