Almost zero chance of finding one in the right spec, but there is the option of getting a sacrificial lens cut for it if you know the diameter and if it has - that thing that screws in the front for which the name eludes me -
I was going to suggest a chap on eBay that will custom cut clear 2, 3 or 5mm acrylic into circles for £3 (posted) per 2 disks. I've used him before and the stuff he sends is great.
Dont know how i would get it to stay in though as the scope is completley one piece no screws hold it together i guess i could just put it on with some glue?