Hello and welcome to the support community.
Indeed you can.
There are two types of disk you could buy. System restore which are gray. These would have been supplied with the iBook when it was new. These would have been Mac OS X 10.4 Tiger or Mac OS X 10.3 (Panther) depending on what year the iBook G4 was sold and which model it is. You can buy these disk, but they are hard to come by. Some online venders do stock them, but you would have to search the Internet for them or eBay.
The other type is a retail disk, these are black disks and have large 'X' logo's on them.
Mac OS X 10.5 Leopard was not supplied with the iBook when it was new. You can buy a copy from vendors like Amazon, but they are not cheap. Apple no longer produce or supply Mac OS X 10.5 DVD's
I hope this helps.
Either the system restore disks or the retail DVD will allow you to put a fresh clean install of Mac OS X on your iBook.
Just one thing worth noting, I don't think it will effect your iBook. The system restore disk that would have originally came with this iBook, had some added software. Mostly iLife which includes things like iMove, iPhoto.
If you install a retail copy, it's just the OS and don't have these added software packages. With your ibook having Mac OS X 10.5.8 already installed, it would have only been installed from a retail disks. Not to say that iLife has not been installed. But iLife could be lost if you install a fresh copy of Mac OS X 10.5.8 from retail DVD. Unless you have a copy of iLife which would be compatible.
Bet of luck.