Dysautonomia my brain vessels constrict and make me feel lightheaded, what drugs vasodilate only the brain?

None, but: Low dose verapamil or diltiazem in continuous release (120-180mg) should help. They prevent vasoconstriction in migraines. Bet it will work!

Vasodilators: There are many vasodilators, but they're not necessarily specific to cerebral vasodilation. They include carbon dioxide, hydergine, papaverine, xenon, vinpocetine, YC-93, dipyridamole, etc. Many have been used to induce "cerebral steal" in the setting of stroke. See a neurologist to discuss the cause of your lightheadedness. http://www.nlm.nih.gov/medlineplus/ency/article/003092.htm