why would you need larger than stock ? the 3120 comes with humongo spikes. i don't know anybody who sells them but you can make them yourself (or a buddy of yours who has a welding machine). cut out the general shape and grind the roughness down on a bench grinder. then drill the mounting holes.
I would have to agree with stihl-nut, my 3120 has got some big bad dogs on it. But, today in Bailey's catalog they are advertising larger after market dogs for Husky saws. You might want to give them a call and see what they got for the 3120.