I'd recommend eBay, too, if you're within the continental US. This might be sacrilege, but after using a TI-89 I shifted to an TI Nspire CAS, since it has one of the highest-resolution screens in the market and still uses AAA batteries – I paid only $25 on eBay.
I was thinking the other day about all the electromechanical devices such as Walkman cassette tapes and portable minidisc players that Sony got to work with just one AA battery as opposed to the usual two.
