had a quick look online and you can get some plain masks for only a few pounds online, that would be perfect for decorating up yourself, and should be a little bit more robust then constructing the basic mask yourself.

few sites i found below

http://www.partydelights.co.uk/themes/masks_domino_masks.asp

http://www.regentgifts.co.uk/blank-masks-c-516.html?gclid=CJvWy-GX8ZoCFZ4U4wodhhPxJg

http://www.fancydress.com/costumes/Plain-Masks/2~325~8

if lots of us are ordering at once we could possibly split the postage too

Good find Rich, I particularly like the phantom of the opera and scary full face ones.

Also some of these masks will be available in party shops in sheffield if you want to see what they look like or would rather pick one up.
There are party shops on division street, broomhill, crookes and eccy road (the ones I can rememner off the top of my head).

Otherwise get some card or foam and cut whatever shape you want. If you're feeling adventurous try shaping further using paper mache.
Check out sources like this youtube video (http://www.youtube.com/watch?v=-e3lATRk3ZE) for ideas on traditional shapes and patterns or just get creative, anything goes.