Hey,
I am currently working on a Mythos Boba custom (you can check the early progress here:
https://www.sideshowcollectors.com/...8544-procrastinators-log-projects-time-2.html ), and while I don't have any parts to sell, I can tell you that you don't need any special material or custom cut, to make the Mythos "side cape".
All you need is a random fabric cut into a simple rectangle, with a hole in the middle of it, for the head to go through. You then put it on the figure like a poncho and then sweep it to the side and futz it down with water, so that the figure will wear it the same way that Eastwood wore his poncho swept on one side in the Dollars Trilogy.
Here's some WIP pics of mine (it's wet in the photos, as I took the pics while I was futzing the cape with water. It dries nicely draped around his shoulders).
This was made out of a simple, random piece of rag which I cut with scisors into a simple rectangle with a hole in the middle. After that all the edges were burned with a lighter, to give it that weathered, tattered look. But at the core of it, it's just a rectangle, nothing more. I will further weather it with dirt and maybe some additional burn marks.