Chris says that 33" are on back order with no release date, or plans to to have any in near future. He says he can't get 31s right now, but get on the lust at Summit if you want before racing starts in March.
I went to the Summit Pro shop, and they said Oct 5th shipped from manufacturer. I knew that was Bullshit, but git my order in. Two days later I was told that expected ship was in November. I ordered 29" front tires last year, and they changed dates for six months then cancelled - so I bought Morosos.
The bottom line is that if you need 31X10.5-15 fir next year - order today.