Q: Where can I source parts for my PowerMac G4s for cheap?

Probably not, as Apple no longer stock or supply parts for those old Macs.

Vintage and obsolete Apple products:

Obsolete products are those that were discontinued more than seven years ago. Apple has discontinued all hardware service for obsolete products with no exceptions. Service providers cannot order parts for obsolete products. These include ALL G4 and G5 models.

http://support.apple.com/kb/HT1752?viewlocale=en_US

As far as PSUs...

You can convert an ATX PSU for use on a G4...

http://atxg4.com/sawtooth.html

http://atxg4.com/gige-da.html

http://atxg4.com/quicksilver.html

http://atxg4.com/mdd.html

http://atxg4.com/

RAM...

http://eshop.macsales.com/MyOWC/?StartOver=1

There are also various user groups online whose users sometimes swap or sell parts. Some forums have classified sections where you may find what you want also.