I use the Maxi-Guard and love it. This is a "hard" plastic chest protector that covers the sternum, breasts and partial sides. It slips into a sports bra-like "cover" so you can remove the plastic and cool down after sparring without the need to completely disrobe.
I've been using my maxi-guard for about three years now and have had no complaints.
ash
The company website, important info on fitting the guard and ordering the correct size.
http://www.qp-sport.co.nz/ This place has the cheapest that I can find in the U.S. This is where I ordered my guard - but they were slooooow.
http://www.boxing-gear.com/acatalog/Boxing_Gear_Women_s_Protective_Gear_489.html Here's where you can buy replacement covers (and they have them in black!) I emailed to ask, and received a friendly and positive answer within minutes.
http://members.aol.com/chajonshim/qpmaxi.html Some ladies really like the cups from Triplette, though I find them to be less that satisfactory (the edges tend to, ah, *dig* in if hit at the wrong angle. Anyway, the link is here:
http://www.triplette.com/sport%20fencing/sport_fencing_chest_protectors.htm We have about 33% female membership in our group. One gal has sewn aluminum plates into her gambeson

Another has the triplette plastic guard that uses elastic straps to stay on over a teeshirt. The rest seem to do fine with just a padded jacket (and of course, when using steel, a steel or hard leather breastplate, just like the guys!)
Feel free to have any of your Lady friends email me with questions,
Ash
ashley@waltrulaw.com