Off the top of my head, Monsterpocalypse. They have awesome monsters, including cthulhus, fish creatures, robots, sentient apes of enormous size, big bugs, aliens, mole men, whatever floats your boat.
otherwise, look around in your favorite online mini shops for big nasty monsters. I've found a lot of normally sized 'big bad boss guys' in the plastic D&D ranges, in super hero lines, and in familiar big names like Reaper and others that would all make perfect city smashing kaiju.
Hydra Miniatures carries the DELIGHTFUL Aquaclops, probably my favorite figure in existence:
They have other big, less cool, not-Aquaclops things as well.
Space Vixens ( http://spacevixensfr...dex&cPath=15_32
Carries some pre-cambrian creatures labeled simply as NASTY BIG ALIEN MONSTERS. The opabinia is my favorite of the two, and both would make neat combatants for Godzilla or whatever.
Toys and found objects+bits would make great giant monsters too.
your bitbox is your friend as well. Try something spontaneous with what you have in front of you - like using a rubber bouncy ball, some green stuff and a few wires to make a woofin' giant eyeball being.
Khurisan has some really splendid creatures in their fantasy and sci-fi ranges, such as the Space Demon Dragon, the Octopus God, and a REALLY badass surreal devil creature who's name is a spin on Baphomet. The stuff is 15MM, but the monsters are pretty big.
Just look around for 'bigger than normal monster' things, you'll be pleasantly surprised at all the options that open up.