I'd probably buy and modify since you're already doing an "original take" on the costume/character. H&M sold a tan coat like that - fitted with a flared skirt a while back. You should be able to find something similar on eBay - just see what you find and compare to how much the fabric itself would cost.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan