I think you could probably get one yeah, but I wouldn't advise it.
You need as sturdy a connection between the bars and stem as you can get for obvious reasons on a mountain bike, and shims will weaken this somewhat.
You can pick up 31.8mm bars for about £30, probably less though, would be a shame to lose the Azonics though, best bars around in my opinion (then again you can get 31.8mm Azonics too...)