These are made for pendants, but you could put a pin clasp on the back and make them double duty.
http://www.alphastamps.com/collageitemspendants.html
These are specifically brooch bases.
http://www.jansjewels.com/other/fbro-1.html
Does that come close to what you are looking for? If not, I'd be happy to look for more if you have more specifics.