In the majority of US states you can order vitamin D testing online and have the blood drawn at a local blood draw center (see my general overview of the process).
I'd suggest ordering from Life Extension because they have the cheapest price I know of ($47), and I've been happy with their service. You can also compare the other 6 vitamin D test providers I've found.
Take a look at my state-specific issues page if you're in AZ, MD, NJ, NY or RI.
ZRT labs has an at home blood spot testing kit for $75. I haven't reviewed its accuracy compared to standard bloodwork, but it might be a good option if you're the type that would rather wrestle a bear than get a blood draw.