Googling has drawn a blank.

Does this exist, and if so does it change the Rotor size in the process?

Is it something like this you want?

http://www.chainreactioncycles.com/Models.aspx?ModelID=13721

You can get them in different sizes 160, 185, 203 are common for avid 140 rear is also reasonably common.

Front and rear adapters are different as you can see form this some what confusing part list on CRC

http://www.chainreactioncycles.com/Models.aspx?ModelID=75477

what size do you want? Front or rear or both?

Merlin Cycles has loads of adaptors and their explanations are pretty clear. Just be aware that Avid seems to have changed their rotor size from 185 to 180 on their latest brakes.
I have just bought some new Elixir 5s and was planning on swapping the swanky hs1 rotors that they came with onto my Stumpjumper, while putting the rotors from that onto the Inbred that the brakes have gone on. But, the Stumpjumper rotors foul the caliper of the new brakes as they are 185mm & I bought a 180mm adaptor.

I need front and rear, but the rotor sizes for Avid stuff 2012 are 160, 180 etc rather than 185, 203 etc.

So I’m after 160 rear and 160/180 fronts.

Unless I’m reading it wrong none of the adaptors on the second link go from Post calliper to IS mount

PM-IS 160 rear is the same as PM-IS 180 front

Lots on Merlin/Superstar

So what exactly do you have? 2012 Avid brakes with 180/160 rotors but no adaptors for either front or rear?

If so, then go to the Merlin Cycles website, then mtb parts>brakes>disc brake spare>2012 Avid caliper mounting brackets.
First on the list is “180mm is front or 160 is rear” – you’ll need 2 of these.

Don’t have them yet, but considering a set of 2012 Avid’s which come with 2x 160, but I have a spare 180 I can use.

@Stumpy Thanks for pointing me at the part on Merlin, just wanted it confirmed that these would fit the post mount on the Avid’s (which you’ve done). Wasn’t sure if the Avids came in 2 both flavours of mount, and the Merlin site doesn’t mention post mounts, hence my confusion.

Stumpy01 is spot on, merlin site is way easier to understand than crc and it is 2 of the top ones on this link you will need

http://www.merlincycles.com/bike-shop/mtb-parts/brakes/disc-brake-spares/avid-disc-brake-spares/2012-avid-caliper-mounting-brackets.html

Do you have mounting bolts for the calipers?

The old avid brakes i have use a bunch of conical washers for 3D positioning or something.

Like these

http://www.merlincycles.com/bike-shop/mtb-parts/brakes/disc-brake-spares/avid-disc-brake-spares/avid-caliper-mounting-bracket-hardware-bolt-kit.html

I don’t know if these are required for the 2012 brakes but I’m sure someone else will be able to confirm.

I just bought some Elixir 5s (2012) from Merlin. They came with 180/160 rotors & the rear came with a PM to IS adaptor, but the front had no adaptor with it. So, I had to buy the adaptor I mention above. Sorry for not direct linking, but for some reason my phone doesn’t display the Merlin site properly.

Mine came with the mounting hardware, as in the conical washer bits, but only one set of bolts (to attach the rear brake adaptor to the frame). I took the other bolts I needed (front adaptor mounting to frame) from the brakes I was taking off.

Hope that sorts it out? Not sure where you are getting your brakes from but Merlin had the Elixir 7s with 160/160 on offer when I was looking. Might be worth a look to compare prices.
For something as theoretically simple as mounting some brakes, it does seem to get complicated quite quickly!