Welcome to Mac-Forums, cuttingadash.
To update your OS X 10.2.8 Jaguar install, you will either have to buy OS X 10.3.x Panther or OS X 10.4.x Tiger. I won't recommend Leopard because:
- I have no idea which Mac your are using and the hardware might not be compatible with the latest OS X version Leopard
- Leopard seems to have a few kinks to iron out still and if you only run older software, they might not get on too great with Leopard: quite a few software apps had to be updated for compatibility issues.
Try eBay or an Apple reseller for Panther or Tiger, I doubt very much the Apple store is still selling those versions of OS X. Just make sure you buy the full retail install disk (it should be a black CD or DVD depending). Another thing to make sure os if your Mac can read from DVDs: if it's an older model, it might not be able to read them. Starting with OS X 10.4.x Tiger, the usual install disk kit as in DVD form and if a user needed the CD version, they had to make a special order at the Apple Store.
As an aside, the newer the OS you will use, the more RAM you will need to add in your Mac if you still want to enjoy using it instead of seeing it slow down dramatically.
Hope this helps.
Edited to add:
PowerBookG4 beat me to it!