I buy used figures, usually at convention Bring and Buy/Flea Market tables. Occasionally directly from collectors who have "graduated" to lead. I'm known in local gaming circles as the guy who will gleefully give a good home to unwanted plastic.
The result is that I have literally thousands of spare figures that I dive into whenever I want to do something out of the ordinary, like convert ESCI Napoleonic French to 1847 Mexicans or ESCI Highlanders to 1859 Austrians.
I also prowl Big Box hardware stores. They frequently carry Department 56 knock-offs, such as random stone roads, stone park walls, barrels and crates and other extras go on sale right after Christmas.