don't know about where to order new cog/tools from, but people say that the hozan lockring pliers are the tool of choice. i'd probably agree, since my experience with lockring wrenches has been realizing that some slip off way easily, and that's a pain.

it's not hard to change cogs. just be careful. clean and grease the threads, and screw it on gently so that you know you're not cross-threading it. be firm when you tighten it down. go for a ride w/o resisting (rely on the hand brake if you have one), mash up a couple of hills, and then re-tighten the lockring.