There are two types of disks available. System restore disk which would have come with your iBook when it was first purchased. These are normally gray and come in a set.
The other is a retail version of Mac OS X. Depending on which model of iBook you own would determine which version of Mac OS X you can install. If your iBook is greater than 800Mhz and has 512meg of memory, you could install Mac OS X 10.5 Leopard. These retail disks are are black and show a big 'X' on them. They are DVD's and can be brought from vendors like Amazon. System restore disk are a little harder to find, but can be obtained from some online apple spears. A replacement Mac OS X disk either retail or restore copies wont be cheap to buy.
Best of luck