They don't make it easy...
Whenever a vehicle is designed, it's very important for the manufacturer to use odd sized O-rings. Forget rounded-up to the next millimetre sizes, you are guaranteed that the vital one needed will be 3.67mm in diameter and 0.89mm thick. Should you, by good luck, find an identical size, it'll probably be of the wrong material.
And don't even get me started on the old "universal" box of O-rings you can buy! These were a great money spinner for someone, but they won't actually fit anything, and you'll be left with a full box thirty years later.
So when it's time to rebuild the carbs on your GS/GSX, it's nice to know that someone has already done the hard work, and research, for you and can supply the complete set for a great price. Without the right ones, your bike will never run properly.
Robert Barr's website http://www.cycleorings.com not only has the right parts, but also comes with tips to help you do the job. I'd already bought some of the O-rings for the GSX carbs, but the ones niggling me were the tiny ones for the mixture screws. I struggled to find the right size, so decided to buy the complete kit for US$16 delivered. Awesome service!