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If it's a zoo with dinosaurs etc, you're going to need some pretty large enclosures, might be worth looking at 1/32 (Britains used to do some, but that was years ago, although you still find bits on car boot sales etc) or larger.Continental firms like Schleich might have a few useful pieces, perhaps even Playmobil - after all a cage for a Playmobil lion should be about right for a 28mm T rex.Model railway pieces could be useful, Preiser do some but potentially expensive, Dapol are more basic, but cheaper. Look for a local model railway show, they often have second hand stalls or even bring and buy (same as wargames shows do) where you can get cheaper bits of scenery - especially if its slightly damaged, but that's not a disadvantage if there's a dinosaur running loose.Pound shops are a good bet for cheap bits too.
Blue Moon have a ncie box of Victorian civilians, you might find some sutiable characters in there.