Where can I get the battery replaced in my third gen iPad

I'll just point out (kind of late) that Apple doesn't actually replace the battery for a specific iPad. The battery service is really just a "battery replacement" where a replacement device (usually in a plain box) with a new battery is exchanged for the old device. They will then send your old device to be refurbished to a central facility for the next customer.

From Apple > Batteries - Service and Recycling - Apple

Call Apple to see if you’re eligible for their warranty for a battery replacement. Good luck!

iPads are NOT part of the reduced price iPHONE battery replacement program.