The IS300 has the Toyota 2JZ-GE engine, this is basically the same as the NA Supra with a VVTi head. The turbo Supra has the 2JZ-GTE engine. Yes, there are many many things available for these engines including cams. All it takes is a quick browing of the Go Fast forum, heck, even the For Sale forum to see what's out there.
This is pretty much "common knowledge" for the people on the site here, so read up - there's lot of stuff here. After 5 years, there are almost 30k registered users here, you can hardly be the first to ask anything anymore.