If you are adventurous, and like to tinker, "Livid Builder" is a line of Do-It-Yourself parts that make it easy for makers to design their own physical computing devices and MIDI control surfaces using buttons, knobs, encoders, velocity pads, arcade buttons, accelerometers, faders, other sensors, and LED’s.

http://www.lividinstruments.com/hardware_builder.php

Midi Botique also sells kits http://www.midiboutique.com/products/products.htm

And also this site http://store.highlyliquid.com/collections/midi-controllers/products/midi-cpu


Duane

Duane,
Maybe a little off-topic but please take a look at this post:
http://roland-arranger.com/index.php?option=com_smf&Itemid=38&topic=1798.0

Could you build something like what I'm looking for? Do you know whether one of these vendors sells a footswitch which has a mode for "double pump" (which would accomplish what I'm seeking)? Thanks!

Hi Ted have a look here for the Midisolutions footswitch controller http://www.midisolutions.com/prodfsw.htm#
the roland VKm8 has NO switch connection to be able to control the Dbeam by foot midi solutions works via midi to let you connect a pedal. I don't know wether the footswitch controller will do what you want but an email to midilolutions you would soon find out they have a large range of different switches. They are based in Canada