Actually, Sheldon Brown suggested that it was fine to use an axle meant for 120 mm spacing when going to 126. All you need is enough axle projecting on each side to key the wheel into the dropout for installation. Once you tighten the QRs, the friction will hold it in place if it's properly tight.

So try just adding a pair of 3mm spacers to the axle stack, and see what you have "showing."

And if you want to buy another axle, Sutherland's says that all Campy hollow axles are 10 mm x 26 tpi (though for all I know, ISO standard 10 mm x 1mm pitch might work fine); for 126 spacing get one 10 mm longer: 136 mm. Be sure it has the keyways (grooves in the threads) for the keyed washers. It's not a big deal to buy an axle longer than you need and cut it down, provided you have a Dremel.