The initial kaiju, for the 60s-70s films, were done in tribute to the art of Shoji Otomo. For the later films I changed the style a little. All the anatomy is based off the 1960s work by Otomo and others in Japanese movie magazines, with some anatomy from actual creatures added in here and there. All the bits of information are culled from Japanese media... you can't make this stuff up!
The Gamera box set sold out in a few weeks but individual films should see a release in the future.