We actually lucked out. Not knowing about these, I had already booked an ADR for Oct. 29th, at opening. My friend messaged me about them on the evening of the 28th (we were still in AZ). Our plan was to go directly to the parks on our arrival, as we had a FULL day of activities. But back to the ADR. When we were able to get in, I asked, and they said they had a few (approx 250). People were buying them up. We were able to get 3 (one for me, one for my hubs, and one for a friend of ours). There was a looooong line outside waiting to get the mugs.
The people inside, who were seated at the table next to ours, asked us about it. We told them, and they ended up snagging one as well.