If your talking about the actual brass power valve that screws into the bottom of the bowl, The only place I've seen them is in some of the expensive rebuild kits online. They are not all the same. Come in different hole sizes. You need to know the size you are replacing (stamped on top). Some also have two holes instead of one.
If you give me the full carb number I can look in my Holley manual and tell you the size. EAC-9510 isn't the full number. Should be another dash and a letter at the end. I assume this is a '52-'53 Merc teapot two barrel carb.
Sal