Tokyu Hands

Polystyrene blocks - more expensive than the real thing ^^;
You can also get wood, fabrics, electronic diodes etc etc. Do drop by if you have the time. Get off at Shibuya station, walk down Center Gai and ask directions from there.
The full article that includes this photo lives at www.dannychoo.com/adp/eng/1280/
You may also want to check out www.flickr.com/photos/dannychoo/collections/ for similar photos