In game when reading the skinning journal, it states that rare skinning reagents, Thunderous Hides and Sunless Carapace, can be found rarely on skinnable mobs. At 25% perception I don’t think I’ve ever seen one of these drop off of a mob that was not a rare creature. I am wondering if this is intended or an oversight as farming these reagents is pain.
We get Sharpen The Knife for a reason.
And yes, they do drop from non-rares WITHOUT the buff.
Since I started doing my supreme lure daily and sharpening the knife at each go, I got a steady stack of those carapaces. 
You have three charges of sharpen your knife.
So if you want to get a lot of rare skins, please try the great stormfang mob, isle of dorn just north of the theater event. The great stormfang is the skinning equivalent of a rich node.
What a lot of people don’t understand about rich nodes (and Seam and EZ-mines) is that they roll several times. Rich nodes roll the main herb/ore/skin twice and because of that they can roll finesse and perception twice as well.
If you use sharpen your knife on these it will give twice the rare hide drop. Similarly using perception or finesse boosting skills like mulch for herbing is multiplied by the multiple rolls on rich herbs.
A second place to look is rares, but only once a day. When skinned they are guaranteed to drop rare hides. I believe elites have a higher chance but not guaranteed.
And last your big summon, your once a day summon, is also guaranteed to give rare hides. You do not need to sharpen your knife on that. It will always give you a rare hide.
I’m not sure Perception actually effects Thunderous Hides and Sunless Carapace
It does. It will be marked with a chat tag. Perception can proc both rare hides and species-specific reagents. It has a higher chance to proc species-specific reagents because its proc chance is (normal chance for item to drop) x (perception percentage).