Where is the expansion valve located

This component is located where the air conditioning line connects to the evaporator. So, at the firewall area. You'll have to discharge the system of refrigerate of course, and remove the engine cover and the air box to access. It's not a long or difficult job --- but it does require the ability to properly and legally handle refrigerants, which most folks do not have. As a result, I would strongly recommend seeking professional guidance for this.

Through testing is highly recommended before condemning the expansion block valve as the culprit for inadequate cooling. Highly recommended!