Hi Johnathan
Does the company that made your axle have and a web site if so please share it with us as a starting point for anybody else who discover they need an axle.
Having a pair made was a very smart idea, over the years I broke both rear axle sides, first being the long side, with help of many friends on MLU a used shaft was found to get my HUP back on the road. Couple of years later the short side snapped. That time no used shaft could be found. So like you I started looking for a company that could make a new one company located in Colorado, US Mark Williams, just check they are still in business.
http://www.markwilliams.com/ I share them as a resourse, 2005 they were very good to work with.
At the time they did my short side shaft I asked about having a long side shaft made and they could not make the long side because they didn't have a heat treating oven large enough to handle the length. They make axles for racing and other high power applications.
Glad to hear your HUP is back to moving under rear wheel drive again.
Cheers Phil