TMAO has been in the headlines recently (including my favorite breathless overexaggeration, "THIS blood test will predict how long YOU will live") as a probably-useful predictor of cardiovascular events.
As far as I can tell, the only company that can do TMAO testing is Cleveland HeartLab. They can send a phlebotomist to you (at a cost of $100-$300), or you can arrange your own blood draw for $35. The test itself is $90, and also includes HbA1c, insulin, adiponectin, and uric acid. It looks like the best way to order is to call them: 866-358-9828.
They provide a sheet with information for your practitioner, including diagnosis codes to help with justifying "medical necessity (things like type 2 diabetes, hypercholesterolemia, hypertension, and metabolic syndrome).
I have a suspicion that the news interest in TMAO may be an example of a submarine PR-generated story by Cleveland HeartLab and/or Proctor & Gamble (who is working on products for managing TMAO's effects). That said, I'd personally toss it in my test regimen if money was no object.