BTW, you might also find this online dealer of interest.
http://www.lensmateonline.com/
A lot of Canon folks over on dpreview.com, including myself when I still had the Canon G3, like what they offer. I bought 3 of their Lensmate adapters for my G3 along w/ a couple filters, wrist strap, etc -- that was a couple years ago. I liked their nicely machined, color matched, aluminum adapters much better than the plastic Canon ones for the G3 -- not sure if it's also plastic for the S3 IS. Yeah, their adapters are more expensive than the plastic Canon stuff, but they seem well worth it. BTW, I bought that many to have one for each lens converter and the CPL so that I could just use the adapter bayonet mechanism for fast and easy changeability -- fiddling w/ lens threads are a pain, IMHO, and could be especially problematic if you go w/ plastic. Anyway, Lensmate.com also offers some pretty good alternatives for tele and wide converters w/ pertinent info to help you choose, if interested.
_Man_