Why a cow? why not a sheep or a shephard. will try asda though. If not i got a black cat costume from halloween, I,ll just stick a couple of white patches on,blow up a marigold and stick it to her belly,teach her how to moo and that should do the trick.
what about buying some 'cow' print fabric. I have seen it in several fabric shops and if she is only 2 you won't need much. She sould even just wear it as a poncho if your making skills are limited