We used quite a bit of D2O back in the day as an NMR solvent. (Mostly we used CDCl3 really.)
You can buy it, it's a bit pricey. But you need to feed it to a person for a few weeks and ensure they don't get any regular water from food or drinking. An ME would be unlikely to check on levels of D in the body, that would take running a mass spec.
A Mass spec is a fascinating piece of equipment these days.