A couple of suggestions:
1. Call Sunfire and ask if they have any old remotes.
2. Check Ebay.
3. Visit
http://www.remotecentral.com and ask there if anyone has one they would sell or know the details of the protocol and commands that remote uses. RemoteCentral and Hi-FiRemote have a large collection of remote codes for all types of remotes and devices. Its possible that someone has uploaded .ccf (pronto) or .mxd (Home theater master) files. The HTM MX-700 can import those codes and build a working setup.
4. Check hi-firemote.com to see if one of the cheap One-For-All (URC-8811) has built in codes for the Symphony. If not, but the details of the protocl and code numbers is known, you might be able to add them to the URC-8811 using JP1 programming. See those remote sites for details on that.
It will take a little work, but i'm sure you can find what you need.