You get them made to order. Or if you're savvy with a welder, buy the metal, and weld them together yourself.
Given the choice I would prefer RT's 8' high 60' diameter flared sides round pen made out of telephone poles, but as I said, we're renting, and there's no point in spending a bunch of money to put in a round pen that we can't take with us when we leave. The landlord would allow it [heck he's actually all for the idea!], no issues there, he would allow us - is actually encouraging us - to put in an arena as well... but yeah, it's a lot of money to spend on a property for little to no return.