You can use a stock LS1 o-ring, then go to harbor freight and buy the green O-rings, that are heat treated and chemical resistant, the stronger ones.
I took the stock one and used a green smaller one from harbor freight and placed it above the stock one on the bottom of the injector. Even though the stock o ring is smaller around the injector site of the intake, The Harbor freight o ring pushes on it so that it will not move. No leaks, no problems. Plus mine is a boosted car. It works well.