You're gonna make me look, aren't you?
http://www.watlow.com/
The 400W figure is a maximum power density, not an actual measurement of the cartridge power. BTW, it's ok to underpower a cartridge (it'll give you more room when the CPU power output grows
).
The smallest heater I saw (and I was barely looking) was rated to 360W. I'm pretty sure that there are smaller ones, but you'd have to contact them for a catalog.