You get a chance of obtaining them each time you run into some weird artifact thingie out in the shuffleboard
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When hovering over such an item, you'd get something like below, which tells you what the "Daily Exclusive" is, how likely you're to get it, and how much longer it'll be offered before "today" ends
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If you don't like what's being offered, you can click "Reroll" to change that Daily Exclusive to some other random item. It requires having collected at least five "Amuni Knowledge" items that day before you can do it. It doesn't consume the items; you simply have to have collected them that day before you're allowed to use that feature.
EDIT: If you're curious, I ended up getting the Supply Windfall 10% out of that chest