Piano Lane – www.pianolane.com 
There is a section of sacred music, and it can be organized by solos or duets.

Marshall Music - www.marshallmusic.com
There is no online catalog, but you can place orders if you know what you want.  Store locations in Grand Rapids , Lansing , Kalamazoo , and more.  They sell music for all instruments.

J.W. Pepper - http://www.jwpepper.com/sheet-music/welcome.jsp 
You can search by instrument and they have an online catalog for sacred music.

One Way Street – www.onewaystreet.com
Provides quality puppets and supplies for Christian puppet ministry, blacklight , children's music, gospel magic, clowning, ballooning, etc.

MusicNotes.com – www.musicnotes.com
Download sheet music.  Includes a section for Christian music.