I guess I was not specific enough -- the Feathercraft seasock I have came with my Wisper. The other is a well constructed homemade one that came with my skin on frame Greenland replica. The replica has a keyhole cockpit that is the identical size to the Wisper -- I know this because the Feathercraft spray skirt that came with it fits the Greenland exactly. So either one would fit your Wisper.
I did dig them out of their storage containers in the basement on the weekend and have to inspect them -- might fill them up with the hose to check for leakage (both have taped and coated seams). Raining out there today (Monday) and I am going out of town early Tuesday until Friday. I will let you know next weekend if either is still functional.
By the way, if neither of the ones I have is usable, Black Dog Kayaks sells several sizes of sea socks for $130. The large size should fit yours (I think the Wisper circumference is around 90" and it fits up to 92" -- my Wisper is not set up at the moment so I can't check it for sure):
http://www.blackdogkayaks.com/seasock.html