It's actually a pretty common issue. My feet are only about half a size apart (15 right, not quite 16 left), so it doesn't cause me many problems. There are at least a couple of shoe exchanges in the country where you could hook up with somebody to swap your unneeded size 10 for their unneeded size 12 or whatever (I did a story on them years ago when I was a newspaper reporter), but I don't remember where or how to get in touch with them. This was pre-Internet, and it was all done by newsletter and mail--the Web should make it easier.
That's not a lot of help, but it's common enough that I'm sure there's a solution out there. For street shoes, anyway. I dunno how many of you differently footed folks are interested in cycling shows.