I plan on eating mine -- the cool thing is that with gold you can do it again and again and again -- they're like everlasting-gobstoppers that you can swallow!

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But to get serious, Gold coins aren't what you would want to use to trade for food anyway (even NOW with Gold undervalued, you'd need to get a small truckload for a single ounce). No, in a SHTF scenario, Gold coins are what you would trade for a house, or a farm, or a factory, machine tools, etc... or transport to another country.
You don't expect to be able to use your 401K account records or large cashier's checks to buy a single loaf of bread at the quickie-stop, or a meal at a local diner (most of them don't even want $100 dollar bills).
My point being that Gold coins are a way of preserving
major amounts of one's assets in an compact, portable form that is not subject to either physical deterioration nor inflation/devaluation. And even then, putting more than 20% of your assets in gold is probably unwise... because gold is not a "performing" asset
(it just sits there, staying shiny ...and exactly the same weight and size as when you plunked it down.)
If you don't already have substantial savings -- such that you have sufficient assets and other items already that would pay or trade for food -- then gold is probably not for you (yet anyway). Better to stock up the larder with canned goods & dried foods (stuff you can and WILL eat regardless of whether TSHTF or not). Then invest in skills, tools, etc. (The guy's advice above on "energy generating" is good -- as is owning land you can use to grow a garden and fruit trees, or raise chickens, rabbits, or even goats on.)
As to WHERE people buy gold coin -- most who do so don't like to answer that (and especially not
on public forums) any more than they like to answer questions about how much gold coin they have, or (even worse) questions about where they store it if they have it.
The best places to buy are local coin shops (which of course change depending on where you live) -- and there are many place online,
Google can help you find some (but I'm afraid you are on your own when picking which company to do business with).
Sorry if that doesn't fully satisfy -- but in some things you need to do your own research and not rely on others to give you easy answers -- and this is one of those situations.