For on-road bikes, have heard the FMF packing is not good.
For my Yosh pipe, I found packing was a catalogue item from Motovan and my local dealer ordered it for $20. Have heard the same for Two Brothers pipes as well.
Watch out which rivets you're sent. I was sent the wrong rivets and ended up going to a fastener supply store for the right ones. Also, for the rivets needed (3/16" SS), I needed to go get a premium hand riveter.