You can buy anchovy dashi bags instead at most Korean grocery stores. the flavor is really light, it’s good enough for ddukbokki but not so good to make Korean stews with. Try to make the stock at home if possible
To reheat leftover ddukbokki: Add a little water in the pot to the leftovers, turn the heat to high just to warm it up don’t cook it. Stirring to make sure they don’t stick to the pan (leftovers don’t need to be stored in the fridge if planning to have it the next day)