Instead, we have to take an ember of authenticity and fan it into a roaring fire of truth. Mr. Flemming probably didn't have a flying car (I'm saying probably because British secret agent), but he still managed to write the heart warming family adventure Chitty-Chitty-Bang-Bang (dirtiest theme song for a Disney movie ever, seriously, think about it).
So, you've decided to write the next best selling spy novel and you're not an ex-secret agent?
Step 1: Pick a theme, something that you can talk to in a truthful authentic way. Whether it's true love, tortured souls, or the thirst for adventure. There should be something about your characters that you can identify with. While you're picking a theme, go ahead and pick up a few tropes. What is your story about?
Step 2: Research. Read a few books on the subject. Read the Wikipedia page. Go to the bottom of the Wikipedia page and check out the sources. Read the sources. Contact your friends--are any of them secret agents?--try and find someone who knows about the topic at hand.
Step 3: Write your novel! (This is the hard part.) Sit down at your computer and bang out 50 to 100 thousand words. Then do it again.
Step 4: Research some more. Go through your manuscript and double check every single fact. How does that gun work? Who's the most important man at the Pentagon? Then check for tone. I know, tone's not a fact but it is a feeling and it's almost more important. If you're writing a novel about a cop and a circus ringmaster then it better have a light jaunty feel (check out my new novel Tempting the Ringmaster for an example!), but if you're writing about a tortured secret agent whose been kidnapped by the enemy then your tone should probably reflect the situation.
If you do the work then there's no reason you can't turn the old 'right what you know' advice on its head and kick ass writing what you don't know.