In the majority of US states you can order serotonin testing online and have the blood drawn at a local blood draw center (see my general overview of the process).
I've always done with whole-blood serotonin testing because of the vague impression it's more actionable than serum. See the next section if you'd prefer serum options.
I'd suggest ordering from Life Extension because they have the cheapest price I know of ($66), and I've been happy with their service.
I haven't found any other providers. Let me know in the comments if you know of any other sources.
Take a look at my state-specific issues page if you're in AZ, MD, NJ, NY or RI.
If you'd prefer serum testing, here's the sources I've found: