Searching on Sourceforge for "java steganography" gave me 6 hits. You could try the same on GitHub, GoogleCode and so on.
(Yea, a plain Google search doesn't have any useful hits in the first page or so. Too many of those useless "download stuff for free" parasites ... pushing the useful links down the search result set.)
The problem is that if you can find an open source steganography library, then so can the folks you are trying to hide stuff from. This may provide them with the clues they need to 1) figure out which library you are using and 2) defeat your scheme.
