New or Used? New (Replicas) can be had from David Silver Spares.
Left: http://www.davidsilverspares.com/CB360G5/part_28432/
Right: http://www.davidsilverspares.com/CB360G5/part_28433/
At $200 each plus shipping, there a little pricey.
I used Emgo's from Common Motor Collective : http://www.common-motor.com/honda-cb-mufflers
They look pretty good and are period correct. Sound a little louder than stock style, but not objectionably loud.