I've got a few different clippers with festive shapes, my favourite being a Christmas tree, though it's not quite as fancy as the ones in the photos on this thread! Most of the novelty paper and card punches you can find in stationery shops or on Amazon can be ordered in the right size, but they don't all clip through ticket stock terribly well, so you have to be quite careful which ones you pick.
I believe some TOCs are stricter than others when it comes to which clippers you're allowed. As a result there is no nationwide standard, and plenty of guards are free to use their own varieties, which I personally quite like!
There used to be different shapes which were very much regional, even job-specific (barrier staff vs travelling ticket examiners, for example).
There are, however, nuances which still apply to which sides you clip the ticket on, depending what message you intend to pass on to other staff who may see the ticket. The best-known of these probably being two clips at either end of the magstripe - look for the excess fare coupon or start asking questions!
