Those of you who are working on costume corsets may find this tip useful.
If you find the prospect of finding a source for boning daunting, you can do what I did with my very first corset several years ago. Not being in proximity to a fabric store, and being unwilling to drive half an hour just to pick up boning (I wouldn't have only picked up boning, and I knew it) I cut up heavy household plastic (like the kind detergent bottles are made from) into the correct-sized lengths and widths and put it down in those boning channels like it was honest-to-goodness boning.
If you're into authenticity or anything like that, well, forget I ever said this. But for a costume for a child, makeshift boning works just fine.
Best of luck!