I always hit up the comic / gaming / hobby stores, especially when traveling, looking for the bulk, undesirable-to-gamers HeroClix figures. One store I went to was selling them for 50 cents apiece. Sprinkling a layout with a few superheroes is always a hit when kids (and us big kids at heart) find them. I've even toyed with the idea of doing a comic / science fiction convention scene and include various heroes, villains, horror and S/F figures as costumed attendees. Currently, my module at most train shows is a "vehicles from TV & movies" theme with a lot of the 1:43 vehicles I've been collecting over the years, and I always put this HeroClix Batman next to one of the Batmobiles on the display:
I also check out the other fantasy and historic gaming mini-figure systems for appropriately-sized items and figures. Although these are a little more expensive, they can often be repurposed. One of the figures I bought was a warrior angel that will look good as a monument in a cemetery.
And 1:48 military models sometimes have figures and items of use. I bought a 1:48 Tamiya WWII generals figure set that contains Patton, Eisenhower, MacArthur, Montgomery and Rommel. The 3 American generals will eventually get painted as statues for a park scene. The other two will probably get reworked and painted as non-military people.