You can also use a small stainless bowl and some heavy wire (stainless preferred, but even coat hangers will work) to make your own as I have in the past. My homemade mushroom anchors fit over the decoy's head and dig in better than any others I've used. The cantilever effect of the wire causes the anchor to tip and dig - which most other do not - and this alone may eliminate your need for going bigger.
To make:
Bend a ten inch length of wire into a "U" and kink the ends (approx. 1/2 inch) toward each other. Get something to hold the wire upright in the bowl and pour in your lead... (I typically held my wire upright with my hand, just make sure you have it protected)... once you get the size you want, you can mark your bowl with a sharpie (for consistency) and punch out a bunch of the size you want in a relatively short time.