of course the GS300 rims fit its the same car just different engines. the IS's will look dumb and not fit right though. what rims do you have on the car now. i have my stock 17's from the 400 on threw the winter just put beefier tires on them and i have my imports from japan on threw the summer. but if your looking for some 16 inch wheels i know someone selling them with tires off a gs300. $400 i believe is his asking price but if you dont want to do that then you can always check ebay. plenty on there. good luck!
~ Stella