It appears that you are looking for a Singer 404 Button Hole attachment for your Singer 404 sewing machine. According to the results, several websites, such as
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sell parts and accessories for Singer 404 sewing machines, including the Button Hole attachment. They guarantee the fit for your machine or offer a money-back guarantee. You can visit any of these websites and purchase the Button Hole attachment for your Singer 404 sewing machine.
It is worth noting that when searching for the Button Hole attachment, you should look for one that reads "Straight Needle" on the fork arm if you have a Singer Featherweight machine, and one that says "Slant Needle" on the fork arm if you have a Singer 301, 401, 403, 500, 503, etc. machine.