One of your current front calipers should fit the rear of your frame by replacing the recessed Allen nut with a normal nyloc nut. Then you just need to replace the pivot bolt in one of the rear calipers with a front nutted pivot bolt. They don't pop on eBay that often, so I've got them before from www.campyoldy.co.uk
They are described as "Brake centre spindle front nut fitting #2012" under the "Stock List" link and are £23.00.
Also bear in mind that a front nutted pivot bolt from loads of other cheaper brakes will also do the job in your Campagnolo calipers and is virtually impossible to spot once fitted.