The students may also use free online dictionaries, e.g. this one or that one.
You may also ask the students to prepare the envelopes as homework. It's pretty useful before an exam! I've tried it once as a revision - the students brought the envelopes having chosen the categories themselves so we had e.g. T-words, Unit 6 words, Adjectives. Some students chose the same categories but it isn't an issue - the words inside the envelopes are never exactly the same!