A/C not cooling as good when stopped?

car would start and ac would be great for few miles, then the fast cycling of compressor would start and would not cool as good when stopped. The shop says expansion valve is leaking and causing the compressor to overheat so they recommend a compressor, expansion valve, and a dryer, which comes to be too high for me to spend on this car. Where is the expansion valve located and how to replace it? I want to try it myself. I have done many A/C compressors and other parts myself before.