Most sugarcraft shops, and some supermarkets, sell sugar or crystallised flowers; mine, here, come from Jane Asher and, in the interests of as much detail as could be helpful, the foil tray I use measures 32 x 20cm and comes from Lakeland, but you could use any receptacle of approx. 30 x 20 x 5cm (approx. 13 x 9 x 2 inches) or 23cm (approx. 9 inches) square.
If sugar flowers elude you, please don't let it ruin your day: just cut the set rice crispie mixture into squares and munch down happily. Alternatively, you could scatter any pretty sprinkles over the mixture before putting it in the fridge to set.
Recipe posted by Nigella