Your best bet would be eBay. Over the past three years, I've seen two bottles of Oldest sold on eBay. I bought them both. Best of luck!
update Aug 5th
I've got a bottle for sale that I'm also going to post on ebay, but you can just it directly from me. I'm located in Singapore at the moment.
http://chicvoyagetravel.com/johnny-walkers-oldest-for-sale/
Johnnie Walker was created in 1865.
Johnnie Walker was born on March 30, 1945.
Johnnie Walker is 66 years old (birthdate: March 30, 1945).
The Blue Label Johnnie Walker whisky is aged for a minimum of 21 years.
Des Walker goes by Johnnie Walker Whisky.
Johnnie Walker Blue Label is aged for a minimum of 21 years.
A bottle of Johnnie Walker Blue Label typically costs around 200 to 250.
nyse:DEO
The most expensive bottle of Johnnie Walker, the John Walker Sons King George V, can cost around 600 to 800.
Johnnie Walker whiskey made its start in Ireland. It was named after the man who first brewed it up and sold it out of a small grocery store. After his death, his sons took over the distribution of the now famous Johnnie Walker.
The price of a 1L bottle of Johnnie Walker Blue Label typically ranges from 200 to 250.
Johnnie walker