You can install any version of Mac OS X up to 10.4 (tiger) on an iBook. When you say they came 'without the operating system' I'm assuming your saying that the HDD's are just blank. So to install you must boot the computer from the install disk you get (I'll get to that in a second) by pressing option when turning on the computer. Then, follow the steps to install (and if necessary format the drive) mac os x.
To purchase an install disk you'll have to check ebay or other equivalent., I'd go with 10.4 as it's the latest system you can get.
Here's actually a link to copies on Amazon.
What is the version of the operating system on this MacAirbook
where do I find that information