I've mentioned before, if anyone actually needs it, I would recommend that people look up and down everywhere and absorb what information comes to them for inspiration for their designs.
I remember a user I met once in IMDB who said that Angels looked a lot like abstract artwork. Looking back I can see what he meant: a lot of the Angels had very interesting and unconventional designs (see Liliel, etc). While I feel that they seem simplistic by today's standards, I think you can see a bit of that abstract tone in the original Angel designs. These things were not your daddy's giant monsters.
That isn't to say that I want people to emulate the original designers and limit themselves to making monsters that were artsy-fartsy ten years ago or H.R. Giger rip-offs. Post modernism, surrealism, abstract art and other weird-for-the-sake-of-being-weird styles are just some of the vast range of things people can look into.
Another source could be, say, a child's imagination perhaps?
Take this drawing of a monster drawn by the seven-year-old girl of one of the people I work with:
Please note the thick and disproportionate limbs and the tubular, jawed mouth. While the untrained eye would see a mess of scribbles, surely the skilled can cull inspiration from this work! Imagine, a disproportionate creature with horrible, ungainly features! How would it move? What dangers does it bring? What the Hell did it do to itself to make it look like that?!
Imagine what other kinds of horrors lurk within a kid's mind - like the following!
Afterwards, the same seven-year-old girl proceeded to draw me twice: one where I'm wearing a bikini and another with a short skirt and a bun on my head:
Please note the artist's depiction of my grotesque body hair and webbed feet in the former, and my frog-like head and terrible fashion sense in the latter. These are surely the most accurate representations of myself that anyone in the Internet will ever see!
So there you have it: though you have a specific goal, you can find inspiration anywhere you go! From military technology, human and animal biology and behavior, and architecture, to literature, video games, anime, music, good trips, bad trips ... eh you get the point.
To be specific, ask yourselves some fundamental questions. How do certain pieces of music make you feel? How do they make you move? What about animals: what do birds and bees do when they fight each other? Maybe military tech is your thing: what natural mechanical processes can be used to simulate your favorite weapons?
Or maybe you're looking at your little sister's doodles: why does the world suck?
