If you're set on replacing the rings, just go ahead and do the whole crank, it's just as easy, if not easier. It's a one-piece crank, I'm betting.
If I was a little bit shady, I might suggest something like going into a Wally World, getting the 800 number off a Roadmaster, and calling them for a replacement crank. Likely get one for peanuts. But I'm not shady, so, I don't know what to tell you.....