Purchase a Gunerra plant and grow one is the best way to get a leaf that can be used. These leaves can become very large and would be IMHO impossible to ship and use for leaf casting. If you are in a zone that Gunnera likes, it will be good to find a spot need a stream or pond on the waters edge and plant. If you live in a colder zone - best of luck trying to grow in a container.

If your interested - Do a search on the various forums and you find lots of post about growing Gunnera.

Mike

Imagine a rhubarb leaf. Now multiply its area by at least 6. Then add stiff thorns. And remember that it isn't flat. That's a Gunnera leaf. I cannot conceive of one being sent through the post.

If you decide to grow a plant give it a space the size of a family car. Here's Gunnera growing in the South West of England. There's a 5' 6" person in the middle of the plant for scale.