The HeroClix line of superhero gaming miniatures are very close to O scale and offer pretty much any Marvel or DC hero or villain you can think of, and often in many different variations of poses and costumes. They are relatively cheap, too, unless you want one of the "rare" variants. They are often available in comic book stores and gaming stores. Search on eBay for HeroClix + your favorite character and you will probably find many to choose from.
The only downside is that you have to remove the gaming bases from them.
Walmart also used to have appropriate sized all-metal super hero figures, so that is another possible source. I don't know if Walmart still sells them, though.
Andy
