Did you ever go to an auction where one of the lots was tickets to something that you knew had been sold out for months? Of course some of those are the result of somebody associated with the organization donating them, but others are simply you being clever about getting your own "tickets".
Board members, committee members and staff who have black, platinum and gold credit cards are offered pre-sale and advance tickets to major events long before the public gets a crack at them. Some even offer a list of what's going to be available in the future right there on the monthly statement. If any of these offerings are a good fit for your attendees, ask them to purchase some and reimburse them. Better yet, get them to "underwrite" them as a donation to your auction.
The other method is to have somebody track the top ticket websites so you can be ready to purchase those hard to get seats when they go up for sale at 12:01 on a Wednesday night eight months before your gala! You pretty much know when your event is going to be held so you can focus on those performances that take place after yours. So by the time your event rolls around you are the ones who "got those tickets".
The other component to consider is if you have an out of state vacation home or hotel/resort donation, see if there are any choice offerings that take place in the same location as the lodging. Together they might just be one of the best auction items of the night!