Inexpensive is a relative term. If you search on ebay the only non-used Clamshell iBook batteries are being sold by one seller who wants upwards of $200 apiece for them. But there is a third-party seller on Newegg that is selling them for $60 + tax and shipping. I got one and it lasted almost exactly 6 hours, which I found very impressive (admittedly, most of the time I wasn't actively using the computer, but I did have the screen set to not turn off during my test). So I don't know if I would say it is cheap, but it's not that expensive either, considering that they function better than most replacement batteries seem to. Here is a link to their stuff on Newegg. They list them by iBook color, but as far as I know they are all identical batteries. Just make sure you don't choose a difference seller that sends them from China.
You mentioned that you got hold of batteries for iBooks rather inexpensively, from where and were they any good?
I managed to get hold of two iBooks and the batteries were rather ok, but if the price is right I can buy a new one!
@pcamen I found my old copy of the original (I think) “Human Interface Guidelines: The Apple Desktop Interface” copyright 1986. I notice you don’t have this older edition on vintageapple dot org and haven’t seen anywhere else online, any interest in a donation?