The Bliptronome Kit converts ThinkGeek's Bliptronic 5000 LED Synthesizer to a fully-fledge Monome 40h. This is probably the cheapest way of getting a 40h (Bliptronic 5000 is $40 and the Bliptronome kit is $73). The kit was created by Wil Lindsay (a.k.a. Wait for VBLANK) and he has plenty of resources on his website straytechnologies. The full conversion took me maybe 2 ½ hours - with 1 hour just putting in the potentiometers (it was a little awkward, which is not the kit's fault).

The Bliptronome's firmware is a flawless conversion of the Arduinome firmware. ArduinomeSerial was running right away and my applications were receiving all the OSC messages without any problem. I changed a few lines in my Processing OscBeatSequencer and it worked.

Seeed Studio used to have almost all components to build you own Monome.

I used the Unsped's Arduinome Shield. I downloaded the design from the blog, separated the shield and the breakout board, and uploaded them to BatchPCB. You can order the Unsped Arduinome Shield for $14.44 and the Unsped Arduinome Breakout for $3.71. These are not my designs and I do not make any profit by offering these - the prices are set by BatchPCB.

Arduinome Shield64 LED Button Pad
IN 16 LED+1
OUT 16 LED-1
IN 1 SW+1
OUT 1 SW-1