Party Shops Novelty shops and stands (like at carnivals) The toy aisle in some grocery and department stores
You may need to use a toothpick or alike to thread some of the string through the small hole. You may also need to widen the small hole slightly. Although removing it should be safe as long as the string isn’t pulled with reasonable force, they are explosives so still express caution while doing removing.
By simply pulling the string from the rest of the explosive. By igniting the explosive directly with fire (instant ignition). By using a fuse (candle wick works) to lead fire down to the explosive.
Get a tealight candle. Remove the wax part from the tin. Remove the wick and the wick base (you can use later this for the ‘Fuse’ method). Optionally, cut back the tin so only a small ’tray’ is left.