[0]: https://www.privatemdlabs.com [1]: http://www.directlabs.com/ [2]: https://www.walkinlab.com/ [3]: https://www.health-tests-direct.com/
I specifically use Life Extension for vitamin D because they're the cheapest one I'm aware of.
Quest-only: DirectLabs, Personalabs
Choice of Quest or DirectLabs: Walk-in Lab (switch the radio button to Quest on the category pages, and make sure a "QD" icon shows up next to it in your cart), TrueHealthLabs (make sure the tests you look at say Quest is an option), HealthTestsDirect (make sure the test list page is toggled to Quest at the top)
DirectLabs and Walk-in are generally cheaper than the others I mentioned.
I just recently have been struggling with coverage of Vitamin D test for kid with Celiac.
I went in, the nurse drew some blood and charged me $150. A week later, they mailed me the results.
None of this was done online, and insurance never was involved. I found a lab in town, asked their cash price, and they told me, and it was fine. Specifically the lab was associated with a larger medical facility, so call around to any local hospitals to see what they quote you.
The "Vitamin D 25 Hydroxy Blood Test" was $47. I got a blood draw order signed by a doctor that I had to take to a LabCorp - so make sure you've got one near you.