Well given the response to 'used riding wear' on ebay I'd stick with that......just make sure they're listed in the 'special interest' section as well as sporting & equestrian
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Other than that tack auctions are good - local organisations such as riding clubs quite often hold them, or if there's any age to them try the bigger auction houses. Cards in tack shops are quite good as well, or contact the hunt - see if they know anyone.