Help with muzzle for a boxer dog?

Well, she's 10, and this winter it's the snowiest ever.....all she tries to do is eat snow!! I need a muzzle to stop her scooping it up. She just can't control herself, and it's making our lives hard work. Can anyone recommend a suitable muzzle? I'm in the UK. Thank you, I'm hoping to brave the snow this morning and buy one.