The only way to get one is to order a new shaft (which comes equipped with the adapter) from Titleist. You can go to any authorized Titleist dealer/retailer to order the shaft. Another option is to purchase a used 910, 913 or 915 and have the adapter removed from the shaft...the only issue here is not being certain that the adapter is actually an authentic Titleist adapter, since some people change shafts and just use an after market adapter.

I'm sure i read somewhere that Titleist don't sell the adapter on its own