A well-established (or old if you prefer) LBS will probably have some extra calipers hanging around. If they don't, see if you can't find a bike at a thrift store that has cheap bikes--one of those bikes might have calipers on it. Still, the LBS is your best bet for it. If you don't mind spending a few bucks, chucksbikes.com has long reach calipers, I think.
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