McCabes usually has one lying around on the floor somewhere broken & neglected
I heartily second the recommendation for the Ali Akbar College of Music. It's a fun place to go but I wouldn't hesitate for a second to order from them long distance. I had a problem with a tabla I bought in person and they dealt with it very quickly through the mail after I was back in LA. They know how to set up the instruments and they carry used stuff as well. Give them a call and find out who to talk to about harmoniums. There are some helpful people there if you can find the right person to talk to. If you talk to the wrong person they can seem a bit flakey though
Check it out. Lots of good info at their site...
http://aacm.org/shop/harmonium.html