I was doing a search in a couple places for cat doggie costumes. Closest I could find was a tiger dog costume on ebay, but I don't think it's what you're looking for. . .
Now I know this isn't a cat costume, but I thought the doggie looked so cute (Skeeter) that I just had to post his link to dogster:
http://www.dogster.com/pet_page.php?i=204199&j=t
Sorry I couldn't of been more help. Good luck!