You need to buy the next version of OS X
Basically, the version you buy will be 10.x (where x is the version). So you have 10.3 (Panther) and the next version is 10.4 (Tiger).
The last number 10.3.x, is updated when required automatically and is free. These are basically patches and 'service packs' created by apple to fix issues in the current version. 10.3.9 was the last update to Panther, so you cannot update this OS any further, you need to upgrade by buying the next OS.
You may be able to get Tiger from an Apple store still, or you might even just want to get Leopard (10.5) as this should run very nicely on your G5 - you can go straight from 10.3 to 10.5 without having to buy 10.4. You may now be able to get Tiger retail from eBay fairly cheaply.