So much wrong in these threads by people who don't know biomechanics. No offense I'm sure you're great elsewhere.
The costal pec is easily targeted by costal flies, or costal presses.
You can't just say do decline bench. Because depending on how someone is anatomically shaped, their decline press can easily be a crap ton of sternal pec instead. (Decline bench sucks because it prevents your scapula from rotating properly anyways and your shoulders run out of room much faster. This is why you see people do that silly shrug movement at the bottom portion of the decline press.)
A fly where your arm path follows along the outside line of your pec will target the costal pec. If this doesn't make sense I would need to post a pic or video.
Dips are also another great costal developer if you understand the arm path needed. Which is the issue here. I can't just tell you a movement because like earlier, what's costal for one person can be sternal for another.
