I think the reason you can't find one is because they're not available to buy. The huge ones on ebay (which are very few and far between) are being sold by individuals who presumably were given them by MAC and decided to sell them on, whether they be doing that rightly or wrongly.
However when they are on ebay then are auctioned in the $1000s if I'm not mistaken. Think how much MAC you could get for that!
Only other thing I can suggest is you buy a standard MAC traincase at a store/counter/online, they are about £200, but look pretty nice.