Where is the fuel filter located on the 2006 kia optima?

Based on the picture below, it's likely located under the hood:

While we find your parts, please enter your ZIP Code at PartsAmerica.com

You didn't specify what Kia Optima model (e.g. EX) and engine size you have.

Like so many late model vehicles, it is part of the fuel pump.
Fortunately, your vehicle has an access plate in the trunk for easier replacement.