Yes it was. The entrance closes after they enter and no one can teleport or use magic to get out. There is an exit in the middle that PCs can get it (which I alluded to), but they need to fly up a couple miles (? that what it says) to leave, so even that isn't easy.
It also says that Wizards get hosed, because they can't get new spells (since they are stuck in a dungeon) and that non-dungeon skills are useless, so players shouldn't take them. (page 12)
Yes, you can break it up into smaller sections/separate dungeons, but that's not what the intention was. Otherwise they would have added an entrance/exit in each section, or allow teleporting out. And wouldn't have changed the XP rules.
Furthermore, the dungeon isn't built like there is free access in/out. The thing was supposed to be a prison. Most of the inhabitants can't break out because the exit is guarded by some really tough stuff (sorta, supposed to be a Titan), so they just stayed and took over their section. Once you break it up into separate dungeons, this no longer really makes sense.
Anyway, as to where to get it, I got mine on ebay. At the time, there was like 20 of them on there, and I got one for $30. This was around the time AEG laid off a bunch of employees, so I wonder if some of them were paid in product and dumped them on ebay or to local stores?