Originally Posted by
sethb
It might take a few months to get an appointment, but most sites handle walk-ins pretty quickly. It's certainly worth checking if you're at the airport anyway.
If you are at the airport anyway, try the walkin. At LAX we just called one morning and asked "any cancellations or open slots?" and the agent was super friendly about acommodating us into a slot that showed as "booked" on the website. YMMV!