Just google them. Crucial has a nice little program which tells you what type of RAM is compatible in your computer. And OWC generally has cheaper RAM overall.
You will need to specify which iMac G5 you have. The 1.6, 1.8 and 2.0 GHz models have two sockets for PC3200 DIMMs, maximum 2 Gb. DO NOT put generic RAM in these machines, they are the fussiest machines made this century.
The 1.9 and 2.1 GHz 'iSight' models have a single slot for a DDR2-533 DIMM, plus 512 Mb soldered on. It can take a 2 Gb DIMM, for 2.5 Gb maximum.
At Crucial you just do a search for iMac G5 and you will see options,
Alternatively, OWC and Data Memory Systems will both have what you need and are among the top recommendations from MR users.
I'll repeat though -- the PC3200 DIMMs for the earlier iMac G5s must be tested and Mac compatible, because Kingston ValueRAM and most generics will crash and burn in that model.