Can I replace the battery in a Mediatek 3351, if so where can I get one ?
No, you can't replace the battery since it's already built-in and it uses an AC adapter to charge it. The battery is a LI-ION that can be recharged over and over again. You will have to get in touch with Mediatek and they should direct you to a local Mediatek service. The battery needs to be replaced by a technician and inserting a wrong one might damage the device, therefore it's not recommended to do this by yourself.