there are so many available pc fans and they will work in your mac also
just make sure that the specs are not lower that the original fans in the way of CFM
here are the stock fans specs
CPU fan on a G4 933:
Sunon 60x60x25mm
12 Volt, 2.2 Watt
23.5 CFM, 4500 RPM (max), 33.5 dBA...