Try IBM spare parts first. We had to order a battery for a 3-4 years old R series (the name escape me), and IBM replacement sold the battery for a reasonable price + delivery.
T series battery listing
I couldn't see one for the T23 - but I would suggest calling them first.
URL for phone and product listing:
#2718218, By Chris_C IBM T23 replacement battery?
Chris_C 457 posts
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Registered 12 years ago