SWR (sewer pipes) are what you're talking about and IPEX makes the best ones. They are upwards of $50 but the bigger ones like 10" diameter, 16' long are gonna be over $150-200. I'm wanting for my backyard winter setup as well but I just gotta keep looking haha. Check out plumbing stores and if you were gonna steal one, check out construction sites where they are inserting pipes into the ground, pretty much everywhere that is being built up.  Hope you find one.