They're "available" in the sense that they exist. The normal sources are parts scavenged from other units. There are three chips in toto -- two ROMs, and one "SWIM" chip. You need all three to enable use of a Superdrive. The ROMs could, in principle, be cloned fairly easily (because they conform to the ordinary pinout of EPROMs of the same capacity). The SWIM chip, however, is a custom mixed-signal IC.