What do you mean by a decent clock? And do you want it on the mantelpiece or hanging on the wall?
If by decent, you mean an old clock that is on the mantelpiece, then try Stokes on MacCurtin Street. They are a clockmakers/repairers and would have a selection of antique (and possibly reproduction) mantle clocks.
If by decent, you mean big, wall-hanging clock, then try Caseys for high-end, or Harvey Norman for middle of the road.