Nyssa, try
abebooks. I've been happy ordering through them.
Not much selection through Amazon's sellers, but there's some:
http://www.amazon.com/gp/offer-listing/0141002689/
http://www.amazon.com/gp/offer-listing/B000HYBT4U/
edit: But then, I don't want The Spy Who Came in from the Cold to win, so... You're absolutely right. It's unavailable. The only option is old, tattered used paperbacks at ridiculous prices.

Seriously, though: The only e-book option I see is the Amazon UK store, for 5.49 pounds. States-side, it's pre-order for a January release.
edit #2: The Inkmesh link at the start of this thread leads to a 5.49 pound ePub from WHSmith, but it's wrapped in Adobe DRM. Does that mean you must install Adobe's Digital Edition (?) software to purchase it?