Order leptin blood testing online
In the majority of US states you can order leptin testing online and have the blood drawn at a local blood draw center (see my general overview of the process).
I'd suggest ordering from Walk-In Lab because they have the cheapest price I know of ($68), and I've been happy with their service. You can also compare the other 5 leptin test providers I've found.
Take a look at my state-specific issues page if you're in AZ, MD, NJ, NY or RI.
Understanding the results
The American Association for Clinical Chemistry has helpful pages for understanding the test and answering common questions.