When I was in my 20s I did lying DB pullovers with a focus on trying to expand and contract my ribcage. Obviously it isn't isolating the intercostals entirely, but if they're weak they should be strengthened a bit.
I supposed you could try a similar movement, but isolate your dumbells at the bottom (extended) part of the movement, and just focus on expanding and contracting the ribcage. If that was uncomfortable for the shoulders you could hold the weight differently or just put down the dembbells and just extend your arms.
Hope that makes sense. You'll probably get better advice from someone else before too long.