Availability of tickets to see Yo-Yo Ma

I am a huge fan of Yo-Yo Ma and have been trying to get tickets to his various galas around the country with different orchestras, namely the Milwaukee Symphony Orchestra and the Chicago Symphony Orchestra. These orchestras, like many other orchestras around the country, only allow season subscribers to purchase tickets to these concerts. The problem is I am unable to be a season subscriber of these orchestras at this time, but I would gladly pay for ticket to any of their Yo-Yo Ma events. I think it is unfortunate that there is not a percentage of tickets that is set aside for non-subscribers. Does anyone have any suggestions on how to best get tickets?