It WAS up on the Diono site - then they took it down. I bet to try to reduce inquiries as to "why can't I buy this seat yet??" Amazon has them in stock on and off - check daily if you are interested.
And you might have a better time with the 2009 though I can't remember when they changed the body style? Was it 2010? If so, then your 2009 is likely to have the same issues as my 2005. I tried reversing the headrest, which would solve the problem - however, the headrests do not lock in when you flip them around
And yes, I think there are only a couple of manufacturers which do not require vehicle headrests for booster seats - Recaro is one of them and can't remember the others. It's because boosters aren't made to be as sturdy as harnessed seats and so the headrests on the booster may not hold up to preventing whiplash injuries in a crash. Though some CPSTs think that it's not that the Recaro is any more "sturdy", but only that they have chosen not to make any statements (i.e. acknowledge the risk) regarding the headrest.
Such a pain...on car-seat.org there is a clear agreement that the Forester (at least the older models) is HORRIBLE cars for car seats. Funny, since we bought the Forester thinking it'd be a great family car. The new ones might be better, but whenever a question pops up there on "should I buy this used Forester?" everyone is screaming "NO, NOT IF YOU WANT TO BE ABLE TO EASILY FIND CAR SEATS THAT WORK!" LOL. And being able to do 3-across in a Forester of these years has pretty much been deemed impossible (though there are a couple who haven't gotten it to work, using Radians...but boosters are a pain because ideally you'd like your child to be able to buckle themselves, which won't happen if it's smashed against another seat).
Someone on car-seat.org said to move our Radians into the Forester (harnessed seats and then use them to the limit) and then put the Cambrias in our Odyssey. But the thought of putting my kids in a dedicated booster in our primary car NOW kind of makes me gag even though I know that data-wise, there isn't any suggesting that they'd be safer harnessed vs boostered at this age/weight.