I went to Russia and took 4 cans of spinach with me in my luggage and it wasn't a problem, the airlines didn't question me. I didn't know you couldn't travel with sunflower seeds, but I would imagine you can buy those in most countries. If you cant get sunflower seeds I guess you could buy a jar of sunflower seed butter and take that with you.
I don't take supplements so I don't have any to recommend.
I think its best to get nutrients from food whenever possible.
If all else fails I would just eat whatever high magnesium and high beta carotene foods you can get while your away. Most all nuts and seeds contain a lot of magnesium.
Carrots, sweet potatoes and winter squash are all high in beta carotene as are many vegtables.