SammyDubzAluminum. Hardest metal out there, you'll never catch an edge.
Lol. You might want to sharpen your edges before hitting an aluminum rail to make sure it slides nice.
OP. it's up to you. Depends on where you live. Check craigslist.I've actually seen some decent rails on there before.
As far as buying new stuff, there's nothing really that great for a good price on the market right now IMO. Actually planning on building and selling some rails for backyard use. Was going to do them this summer but I traveled a bunch and in the winter I'm busy building at ski areas.
If you have a friend, friends dad, uncle, whatever who welds that's a good way to go. You can order the steel right from a company. Can look up online or ask around for places in your area. If you're on the way, local, or order enough steel sometimes delivery is free. If not it could be $100 ish or you could pick it up there. Just make sure you have the means of securing it. Rope or ratchet straps is probably your best bet.
Honestly with a cheap sawzall and piece of shit welder you could build something good enough for your backyard. At the same time even the cheapest most garbage welder will run you close to $100. But then you still have the welder at the end of the day. If you go that route though you'd also need to buy an angle grinder. You can get a shitty cheap one from harbor freight for $14 but they suck and you might get 2 months, or 2 hours out of it. You never really know. On the plus, home depot just started selling a ryobi grinder for $29. It's not awesome, but it's pretty decent, and for the price it's hard to beat. I just grabbed one for work. Didn't want to be into it ore than that in case it breaks or gets stolen.
For under $200 you could get enough stuff to build some rails not including cost of steel.If you were going minimal as fuck you could do it for under $150.
Steel depends on what you want. You can look at an online metal shop to get an idea of the pricing. You might be paying a little more from your local place but it'll give you a pretty good idea.
If you can, scrounge up some scrap steel for legs and feet. Then you only need to purchase the actual top part of the rail that you'll be sliding.
As far as sizing, 8th inch is really all you need. I wouldn't go thinner than that, but you don't need to go any thicker either. Make sure your legs are spaces decently. If you're running 8th inch and have 10 feet at the end of the rail after the leg, it will bounce a bit.
Another option is to build a 2x4 frame and bolt steel to that similar to pvc. I would bolt that shit on proper though, and get it nice and snug. Instead of the lose feeling like some peoples pvc rails, it will be rock solid, and it won't ride any different than a proper metal rail.
Idk, if you like to build shit, have a few bucks, and like skiing rails enough to be using them for a few years, getting a cheap welding setup isn't a horrible idea. You can build stuff for yourself, and learn how to weld in the process. Not a bad skill to have. Also the nice thing with buying tools is, if you need it you have it. It'd nice to pick up a new tool once in a while. Then eventually you have a bit of everything, and if you need something you have a drill, some saws, grinder, welder, etc and can get things done.
I'll shut the fuck up but hope there was something helpful in there