If you go to the events you might find shops there selling all sizes? There will surely be places to purchase clothing at the different events?
For anyone going, I've just found this site selling things like Union Jack earrings, flip flops etc. Awesome idea http://www.ukactivitystore.com/olympic-special/
Other ideas for showing support are Union Jack earrings, Union Jack pendants, flip flops etc. There is a good selection on here:
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