I play Moosewood mouthpieces.
http://www.hornmouthpiece.com/ If you just go to the website, it might look a bit eccentric, but the mouthpiece maker has an impressive client list. I would suggest sending him an email telling him exactly your wife's needs (I'd include the make and size of her trombone mpc as well for comparison), and see what he suggests.
When I was getting a new French horn, I called him up, told him what horn, what my background was, what kind of playing I was doing, and particularly what kind of sound I was after and his suggestion of a mpc was spot on. Also, his costs are very reasonable, something to consider when the mpc hits the Customs Import office.
(an aside - once when I had an alto sax mpc coming in to the Zoll office, the official had me open it up and show it to him. He asked me what it was. I turned it up so that the large opening was facing upwards, toasted him, smiled, and said, "ein Schnappsglas" (a liqueur glass). He snickered and motioned with his arm for me to leave, LOL. I didn't pay anything because he got a kick out of that. Who said German Bureaucrats are all sour?)