B&H had Canon's lens adapters for the S3 IS in stock several weeks
ago, as well as the required adapter tube (LA-DC58E), which includes
a lens hood.  Having a wide-converter for my Fuji P&S, I only got
Canon's tele-converter (TC-DC58B) for the S3, and it exceeded my
expectations, which only was that it perform well optically.  It
doesn't appear to degrade auto-focusing performance at all, and the
IS continues to work well.  When used, note that there's a menu
option where you tell the camera which lens adapter is mounted.  The
adapter will produce extreme vignetting if the lens is zoomed to the
wide end of its range, which shouldn't be a problem, since you see
it in the viewfinder (or LCD display).

 I'm pretty sure that B&H ships to your location, and Adorama
should also carry these items.  Both are very reliable companies,
and their prices are usually quite good.  I believe that Adorama
also sells a non-Canon adapter tube for the S3.  You can check for
prices and availability at www.bhphotovideo.com and www.adorama.com