Maybe Shane C still has many sheets of 1/8 " available from when Home depot cleared out there stock. I bought mine from him many years back.
Flush mount windows are tapered on the edge . So you would have to set up a router table and flat bit, to taper it, or be happy with the 1/8" sticking out. 1/8 is abit small to use with window rubber, but it does work. I used window rubber to hold in my front window.
Kevin