If you want wood you will most likely have to have it made to order.
If PVC or Polyurethane is OK you could try Outwater plastics. They
sell flexible moldings of many types and may have something that will
work for you.
Window & Door section; last 3-4 pp or so...
Several others as well...somebody else mentioned Outwater.
The manufacturer of the window unit itself is another bet...
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