I get all my stuff from ebay! Plenty of people selling G5 parts there. I have used the iMac G5 20in without the HDD brackets before works just fine. Just make sure you don't leave out the temp sensor and stick it in a good spot for it to read the temps. If you leave out the sensor I guarantee you will have major graphics issues almost immediately.
Also try craigslist, there are usually people on there selling broken "as is" G5s and parts for them. Also pay attention to people who post ads saying that they buy broken or not working apple computers. Write down their phone numbers and ask them if they have the parts that you need.