the ones i have came from pets unlimited and are called 'quick clips'
i found them in the bird section. they're not ideal, as they're designed for bird cadges, not aquarium tops...also, i was JUST in another thred somewhere where 'binder clips' were mensioned (i belive it was robbiescornfield, was that you?).
really, any sort of clamp that will hold the top on would work. have you looked at local hardware stores?