Or perhaps trace amounts of residual TARDIS energy? And being a device with an origin from inside the TARDIS, it's made with similar technology and properties. If Amy and Rory can make a Timelord baby, I'm sure the TARDIS has other tricks up its proverbial sleeves. It may not need a chameleon circuit to change appearances, perhaps it's a natural evolution of a product of the TARDIS that, with so much traveling through time, loses its exact, current state of existence for brief moments, or weeks on end. TIMEY-WIMEY.
But now we're just over-thinking things
If they put production on hold to seek out or build a perfectly accurate version of a prop every time the original gets misplaced or is currently unavailable, and didn't make use of stunts, alternates, past versions, replicas, etc. then precious time and money would be wasted left and right. I mean, it's not like it would happen a lot, but when David Tennant is on set, I can't imagine some shmuck from the props department interrupting a shoot because the sonic they're using doesn't have the right shade of crackle or is 2mm thicker in diameter.
"Uh, can you wait here for a few hours, David? We gotta get props to bang out a more accurate sonic, this one's the wrong color...and the slot for the slider doesn't go to the second ridge. People from the RPF will notice, it'll be a huge embarrassment to us and the BBC, not to mention a disgrace to our craft."
