This will be moved to the
Disneyland forum, but I might as well answer you.
MTTMM is only part of packages through certain companies. That said, sometimes people don't want them, and there is a thread on the Disneyland forum with a long list of people with the same request. When someone does have a voucher and knows about the thread, they offer it to the person next on the list.
There is also ebay, but seriously, we were even the "family of the day" during MTTMM when we had a package staying at Paradise Pier Hotel, and it was NOT any sort of thing we'd pay extra for. It's just an hour with smaller crowds, but you still wait a long time to meet Mickey etc unless you skip the tiny ceremony and just go get in line at Mickey's house, and since there are only two rides in there, one with Fast Pass, it's just really not that huge a deal. But as I said, some people do sell and buy them through ebay.