They're not going to be able to because they were most likely custom made. Pirate pants would be the closest you can get to these.
https://www.zootzu.com/collections/mens-pants/products/copy-of-baggy-pirate-pants-solid-cotton?variant=42976807780467
That's not what they are though. I'm gonna try to explain why this would be an impossible task.
I've fought every medieval combat sport there is over 20 years from Belegarth to Dagohir battle of the nations and SCA. These are garb pants and they come in a wide variety of styles. They're not hosen style because they're too baggy or he has the ankle binding tucked in his boots. How one acquires these type of pants is by having them commissioned. They'll be made of durable material using a pattern developed by fighters for fighters. They usually have things like a reinforced crotch.
You can get this billow effect with hakama, wrap pants and harem pants.