In the majority of US states you can order selenium testing online and have the blood drawn at a local blood draw center (see my general overview of the process).
Here are the companies I've found that allow you to order selenium directly:
Take a look at my state-specific issues page if you're in AZ, MD, NJ, NY or RI.
Chris Masterjohn has a podcast episode about selenium testing that was enourmously helpful to me. There's also a transcript if you're not the podcast-listenin' type. Here's my criminally-oversimplified mind-map of it:
I haven't yet figured out if there's a way to get the Genova Oxidative Stress 2.0 panel he mentions without a doctor. In the meantime, Genova offers a doctor finder.