The expansion valve is near the brake booster where the 2 AC lines go through the firewall.

The valves are in the $ 25 range for aftermarket and I just change old valves anytime the system is empty to avoid future problems. Access can be difficult though but removal consists of one nut for the hose , two allen / torx bolts and the valve comes off.

With a properly operating valve, the suction hose ( the larger one ) will be cold as it exits the valve, if it is hot and the system has a proper charge the valve is sticking shut.

Make sure the compressor isn't stopping though. And check to see if the heater valve is opening when it shouldn't. There was a recent thread where this happened and one side of the dash vents were hot and the other cold during AC operation. http://www.peachparts.com/shopforum/tech-help/145624-hot-cold-air-same-time-c280.html