I hit the discount stores. Plastic dinosaurs, fossil dinosaurs, giant spiders, snakes, frogs, lizards-- for stuff like that you can get an army for a few bucks.
Last game my group fought a mass of Mage Knight and Heroclix monsters, a bunch of large tarantulas from a Halloween party favor pack, a Huge spider from the Little Miss Moffet MacFarlane line, and a Colossal Spider played by Shelob from LOTR. There was even some DDM's mixed in there. The whole battle took place in ruins represented by aquarium terrain and a plastic temple from the old Dark World board game.
Basically, anytime I see something cool in a bargain bin, I nab it.