This one got my brain cells working. I know I've seen them in novelty shops in the past, and when my boys were small I used to get things like this for them out the Stockingfillas catalogue. However I've just looked at their website- no "moo boxes". A google search of "moo voicebox toy" brought up this site
http://www.dooyoo.co.uk/offline-shopping-misc/ bear-factory/1022090/ and in "choosing a bear" it says "can add a voice box to your bear! They come in a bark, moo, meow, "I Love You", etc or you can record your own message...these are located next to the stuffing machine and cost �3.99". If that doesn't work for you I also found
http://www.toyrepairs.co.uk/prices.htm . Their teddy growler is �9 , but they might be able to point you to a supplier of "moo voiceboxes" ?