Just had a chat with Selectronix about their acoustic vents: http://www.selectronix.co.uk/acoustic-vent-patch-10x6mm-black-7kpa-c2x26001602 The minimum order they would be willing to consider is 500, so it would need a good few bat enthusiasts to want to get together and make a joint purchase, don't know if anyone on here is skilled at setting these things up on group gets or similar? The chap I spoke to said that once their vents are attached they are attached and don't generally need replacing, he also said that any vents purchased would need to be used (stuck) within 6 to 12 months as the adhesives start to degrade if they haven't been used after that.