yes, it's quite difficult to get birkin in the Spore and HK boutiques,you may try boutiques in other places like Paris, USA and Tokyo; and if you don't mind paying a premium you'll probably score one in Tokyo easily. Read up and do some search on the Hermes Main Forum, they are a lot of valuable info that will help you in your search for a birkin 
http://forum.purseblog.com/hermes-shopping/newbies-how-do-i-get-birkin-other-bag-505926.html

http://forum.purseblog.com/hermes-shopping/newbies-how-do-i-get-birkin-other-bag-505926.html