Where to find a 14.4 mm long and 16.9 mm wide hub cone?
If you visit your local bike shops, there is a slim chance to fund a replacement. On the other hand, the most reliable and time-efficient solution is to replace the whole axle. In my region, the options are an Weldette unit for about 7.5 EUR or a M-Wave axle for about 2.5 EUR. Do ask around.