I must of overlooked it in the Cave of Gi and was wondering where I could possibly pick up an Added-Effect materia?
The Cave of Gi on Disk One is the only place to get the Added Effect materia. If you miss it, sold it, or otherwise got rid of it, there is no way to get another.
See here for more information.
There are unsupported rumors that you are able to dig one up in Bone Village at a later time, but I've never seen conclusive evidence that that's the case. No reputable sources on the web support that claim.