To find a suitable paper, you may have to do some experimenting. Usually black construction paper is used for silhouettes, but since you want to decoupage the silhouettes you want thinner paper with minimal ink bleed.
If you can’t find a paper that doesn’t bleed ink when brushed with decoupage medium, this article on Decoupage Short Cuts contains two ways to seal the ink first.
For more on decoupage, visit The Decoupage Index Page.
Recommended Reading
These books are excellent resources for decoupeurs. Not only do you get tips and techniques, but also artwork that you can use in your own projects.