So after about an hour researching the public domainess of these comics, I was able to determine this:
Plastic Man and BlackHawk are definitely NOT in public domain. Yet the issues of Police Comics, published by Quality which featured Plastic Man, apparently ARE in public domain.
However, DC has trademarked Plastic Man, which might make these comics problematic. So if someone at DC sees this, don't flip out. Just email me. I'll pull these from my site. I wouldn't want to scuttle any movie deal you guys might be cooking up with the character.
[ Police Comics 28 ]
The real reason I'm posting these comics is because Police Comics also featured the adventures of The Human Bomb, and that guy rocks.
Here's what wikipedia says about The Human Bomb:
[ Police Comics 36 ]
Note: If I have to pull these comics, you will most likely still be able to get them at http://goldenagecomics.co.uk/ which is a great place to get public domain comics. Check it out!