That lockring is by no means proprietary. Any old lockring tool will work, and is an essential tool to have around, especially if you plan on switching out cogs yourself. I have the park tool one that Panthers posted but I liked the Hozan ones I've used, they are thicker so they engage the lockring better and don't cut into your palm like the park tool does.

Also, when you replace your cog, or whatever you are planning to do, tighten the new cog and lockring as tight as you can, then ride up a big hill or sprint as hard as you can to fully tighten the cog down, then tighten the lockring again.