I couldn't find any for a normal looking padlock design.

Here's a stamping plate from Maniology with a stylized padlock and key: https://maniology.com/collections/nail-stamping-plates/products/happily-never-after-sweet-revenge-xl192-single-plate

Or you could try using this plate with shapes to make one: https://maniology.com/collections/nail-stamping-plates/products/1pc-xl-nail-plate-shape-nouveau-back-to-basics-xl-501

But the closest I think I found were nail decals: https://www.amazon.com/dp/B00JEHWAIA/ref=cm_sw_r_cp_apa_fabc_pIl.FbGYG0ZR3

If you want a standard looking padlock, it'll probably be easiest to use the shapes plate. You can use a dotting tool for a keyhole if you want to add that.