I have used DTS Midi Systems encoders for a couple of projects, and they have been very reliable.
http://www.dtsmidisystems.com/midiencoders.htmlYou have your choice of positive common (MIPC-1AHV) or negative common (MIPC-1A). The encoders are simply programmed using DIP switches to choose the midi channel and starting note number. The same encoders can be used for stop tabs and pistions.
There are auxiliary inputs, as well as four analog inputs per card (5000 ohm linear potentiometer.
If you have SAMS or solenoid-operated draw knobs, the IDC-24 and IDC-48 encoders/drivers will encode the stop switch and also drive the on/off solenoids. It even has an auto-test feature to operate all stops sequentially.
If you go with DTS cards, purchase the Molex crimp tool for making the connections. If you opt for the MIPC-1A, the board has two 32 pin ribbon connectors as well as Molex headers.
Ray