Anything that doesn't interact with the bumper, hood, or fenders will fit. Otherwise, there might be some customsing needed. But anything mechanical for the 2.5RS or WRX will work on the Aero or Linear.
to my knowledge, there is only one that will fit with no customizing. i can't remember the name, but search on nasioc.com in the naturally aspirated forum. should still be somewhere near the top. injen or somebody. for 05 subaru went to the maf/map(whichever one they didn't have for 04, my memory is not what it once was), and since the aero/wrx is here, there is less interest in a CAI. the CAI's from 04 and down won't work from what i've been told.
I'm guessing that, with the redesigned front end/sheetmetal, this probably changes things insofar as fitment, however....
On the Subaru end of things, the one most highly regarded is Cobb Tuning's CAI. It's very expensive, though. www.cobbtuning.com
Injen is popular, as is the K&N one, which you can buy in lots of places, but here's one: www.subaruwrxparts.com
There's also GanzFlow and PDM-Racing, which use a similar design, with a different shape. PDM uses' Ganz's ver.1 design; Ganz then changed the shape to their current, or ver.2 CAI. The PDM is also about half the cost, at $50 USDM brand new (they're in Canada). www.pdm-racing.com I have the PDM in my car.
Can't say anything about fitment, but those are the ones I know of. There's also the Gravelsports unit, which looks more like a TMIC, mounting with a panel facing up towards the hood scoop. Search www.nasioc.com or www.RS25.com for info on those.
Group buys happen occasionally.