I bought a sports stopwatch from a high street jewellers - 14 quid - much better value than the 40 quid for one from Transair I have to say (although it doesn't go with 4 bar epaulettes as well
). To get it to attach to my kneeboard, I just Araldited a smallish bulldog clip to it (I think WHSmiths sell them as letter clips). Works a treat.
A mate of mine who is still doing his PPL at the moment has a kitchen timer, but he said that the buttons are so big that it is very easy to hit one of them and stop, or reset the timer - DOH!