Are you talking external battery packs (in which case they are all non-Apple as far as I know), or are you talking about the batteries which are inside the iPod sealed
unit? If the latter, then the iPod is a sealed unit, so you probably aren't going to replace them very often (my battery has lasted 1.5 years so far and still going strong), but when you do get around to replacing them then you can choose third party merchants if you don't want to purchase from Apple.
Regards, Andrew