I think most of those 'officially' recorded by the orchestra or organization probably never see the light of day again. However, there are a great many enterprising individuals (aka 'pirates'

out there who make 'live' recordings from the audience, Some of them make their recordings available to interested 'fans' (either as a kindness or for a price). They're not ALL in the business of SELLING 'pirate' recordings (and it would be a pretty slim living, if that were all they did). However, many are willing to trade for recordings they do not have, and most will make copies for you for the cost of the disk and shipping charges. Just post a request for a specific performance, with a valid e-mail address, and you'll probably hear from some of them.