The easiest and cheapest way is to go to a local metal/welding shop and get them to make something. Go in with a plan of what you want, but consult with them to make sure you get the best quality and price. Ask them things like what lengths the metal comes in. Making a rail with these lengths can potentially make it cheaper, because they won't have to cut the metal.
There are also various places that specifically make them. These however, are rare and expensive. They know what they're doing in a ski sense though, so you do get some value for all your money. The trick with this though is that you pretty much need to find one locally. Search the internet and check with your local mountain to find out where they get theirs. They are out there, you just have to find them. If you do get one from someone like this, try and find somebody who can deliver for a low cost, or that you can go pick it up. Rails are big and heavy. They are NOT cheap to ship.
I would strongly recommend the metal shop method though. It's gonna be cheaper, it's less hassle, and you can get exactly what you want. Probably the easier solution.