Check out www dot aircondition dot com.
Theres tons of information on there about trouble shooting ac systems.
Im sure with some troubleshooting following the guide there you will find a faulty compressor of a plugged orifice tube.
The main thing is to either find the root cause yourself or keep looking to find a shop that will troubleshoot it properly and not just throw parts at it. AC systems will get expensive fast and just replacing this or that may not do anything at all except cost more money.