I have a virtually unused pair of Bontrager 48cm Cyclo Cross bars and adjustable stem that came on my Trek T2000 going spare. (see the picure in the 'my tandem' thread below). If you're in the UK I would be happy to sell it to you for a few £s, but if you're in the US, the postage probably wouldn't be worthwhile, so visit your local Trek dealer who will be able to order this bar. Also when changing don't forget to put some lengths of old brake cables into the grooves (Trek and probably other manufacturers don't) it makes the bar much more comfortable.
As my wife's bars are relatively low and forward and my wife's shoulders are narrow as well, we thought a 42cm ITM millenium bar I had lying around would be better. It works fine, as long as I don't try to reach too far to the side when putting a foot down.