1996 Chevy Blazer fuel tank

I removed my fuel tank and am replacing the fuel pump. The problem is that the snap ring locking the fuel pump to the tank was not in good condition and I wanted to replace it. Every auto store I go to has a lock ring. This is ok, but the one all the stores does not fit my truck. Lock rings are 4" o.D. And the snap ring I took off is 4.5 i.D. Any idea where I can get this ring, and why did they use the snap ring instead of the lock ring? Even my service manual says it should have been a lock ring. Very frustrated in Mass.