You can find info here, as well as shop for options. Think well whether you will actually use a tele converter or possibly even a wide angle converter. Note that Canon does not offer a wide angle converter option for the G10/G11/G12 for a reason.
I have the Canon 2.0x tele and 0.75x wide converters for my G9 (designed for this camera), and have used them both on my G11. The wide converter does not work well on my G11, I get a lot of blurring to the left and right sides of the image for landscape shots, but it works well at macro distances. I therefore do not use it with my G11. Others have used the wide converter on their G11/G12 with good results. The tele lens works fine, but I don't bring it often, due the the size and weight.
You should also consider just getting the Canon filter adapter if you are only going to use a filter and/or close-up/macro lens. This adapter zooms out and in with the lens, so you do not have to be concerned with vignetting using 58 mm filters. It will not work with a tele or wide converter lens. You would have to use something like the Canon or Lensmate adapters for a tele or wide lens.