I will urge you to start the D3 right away, without waiting for a lab test. I promise you that unless you have been taking regular D3 supplements at high levels, your D is low. (If you had a thorough annual physical (Medicare, I assume), your D was probably tested then.)
The best way, as you say, is to ask your doctor to prescribe the vitamin D test. If you don't want to go that way, or get refused, you can have the test done at a lab without a prescription.
Your doc should be okay with ordering the test, just in general because most docs appreciate the importance of D. Also, if s/he has any respect for what you are experiencing with CH, it seems like it ought to be enough to say that you have seen some promising results from an anti-inflammatory regimen that includes D3 and you'd like to try it. (I would try to avoid telling the doc what the amounts of D3 in the regimen are, because some can get a little freaked out by that.)
For the lab test, if you just google, lab vitamin d test near me, you should see places where you can go to get the test, and pricing, etc.
There is also a mail-in test that you can order online (or maybe find at a pharmacy). I think this is the least reliable (and slowest) method.
I hope you are splitting your Imitrex injections. Getting partial doses from sumatriptan injectors (Imitrex, Imigran, etc.) - ClusterBuster Files - ClusterBusters For a pill to have any chance of helping, it has to be taken at the beginning of -- or before the beginning of -- an attack (some people can do that if the attacks come at predictable times).
I'm sorry you don't have O2. Some people get good abortive results from deeply inhaling cold air, from an air-conditioning vent or outside, if it's cold out.
Please get back to us if/when you are considering busting. This isn't the right board for discussing it, so I'll just say that there are substances that you can legally buy and possess that are just as effective as MM (eliminating the sourcing problem you mention). And they are typically tripless.