@onbehalfof , there are a few supplements with vasodilating properties and also precursors, although these might also prove difficult for mast cell reactions. I’ll have a look for some recent threads and post links.
If you have orthostatic intolerance, it can be a bit of a juggling act. On the one hand, you want to squeeze the blood supply upwards (with vasoconstrictors, compression garments, or by increasing blood volume), and on the other hand, you want to enable blood to flow into the brain easily (medications or strategies that increase cerebral perfusion).
So you can find yourself in the odd position of wanting to vasoconstrict your lower body and abdomen and vasodilate your head!
You can do the first part by wearing compression garments on the lower body and increasing salt and fluid intake (or having a sachet of oral rehydration solution).
Then you could add substances that increase cerebral perfusion without specifically being vasodilators, or substances that tend to affect the brain more than the body, although I can’t think of any right now. Maybe something that’s anti-inflammatory or antiplatelet? I am sure other folk will have ideas.
