Are you referring to a Resonator? This is usual installed after the catalytic converter and before the muffler to help eliminate raspy or hi frequency sound.
Canadian Tire does sell this item, (universal bullet style) ask for a resonator instead.
Are you referring to the style of muffler that utilizes an insert
to eliminate hi frequency sound.
I would assume you would need to visit a performance muffle shop for this.
Here is an example of a factory resonator from a BMW E46
Hope this helps.