Anyone know where to get plastic 4x4x6 lumber?

Looking for something similar to this:

https://www.menards.com/main/building-materials/landscaping-materials/landscape-edgers/4-x-4-x-8-plastic-timber/4x4x8brpl/p-1454045334258.htm

Want to make a fence with it.

If I could get it locally that would be awesome