What you're probably looking for is a bass pedalboard for an organ or keyboard.
For example, Hammond-Suzuki offer 13-note (XPK100) and 20-note (XPK200) versions. These are MIDI controllers with pass-through, so they can be used to control any MIDI keyboard or synth module.
http://www.amazon.com/Hammond-XPK-100-1 ... 0002D05B4/http://www.amazon.com/Hammond-XPK-200-2 ... B0056IBK0K
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Vivat diabolus in musica! MTGuru's (old)
GG Clips /
Blackbird ClipsJoel Barish: Is there any risk of brain damage?
Dr. Mierzwiak: Well, technically speaking, the procedure
is brain damage.