The Intel® Centrino® Wireless-N 135 is in fact considered a legacy adapter, but it is still in a "Launched" state.
Since it was released some years ago, it is possible that resellers are now focusing on newer adapters. As we understand, it is still available from some online retailers, or you may contact a local reseller using the Intel® Technology Providers Locator.
If you are not able to locate it online or with ITP's, we would advise to post a new thread in the The specified item was not found. communities, so you can ask for possible substitutes to be used in the Intel® Galileo Gen 2 Development Board.