I rarely use collars, for the olympic lifts they are not really necessary, unless you have a big stack of plates left and right---then it may happen, during some triple snatches or clean & jerks that you loose one of the plates. But even here spring collars usually work just fine... Any used collars work fine too. If you buy new, Ivanko collars, for example
these are a good buy. Very sturdy and they really hold the plates in place, which is not a given with cheap collars...