[mark844]

I'm assuming that you are referring to a REB1100/GEB1150/ETI2.

Bad news:
1. You won't be able to buy a "standard" replacement battery - no-one makes the device.
2. The batteries are not like a remote - you need to open the device to access the batteries.
3. The charging/protection board is attached to battery pack and needs to be saved for re-use.

Good News:
1. The device uses 2x 18650 Li-Ion rechargeable batteries (in series). These are very common and a cheap source is eBay. Any capacity e.g. 2400mAh or more will work - the larger the capacity the longer they will last before needing to be recharged. You will need ones with solder tags though. Another source is old laptop batteries as these are commonly used there as well.
2. You will need simple soldering skills or know someone who has.
3. you will need basic electrical knowledge (battery polarity etc).

The method:
1. If present, remove the memory card.
2. You need to remove the 5 screws on the back of the device.
3. Removed the back cover carefully - the battery is secured to the back cover with the battery cable going to the motherboard on the front cover.
4. Un-clip the battery connector from motherboard - pulls out.
5. Remove 4 screws (2 clips) holding battery.
6. Carefully remove black insulation around battery so that charging board can be accessed. This board will be re-used with new batteries.
7. Make notes of battery polarity, wire locations etc (take photos)
8. Cut/desolder the tabs from the batteries where they connect to the charging board.
9. Connect new batteries - refer to notes you made
10. Use insulation tape to wrap new battery pack
11. Connect new battery to motherboard and test that device turns on. Turn off after testing.
11. Re-assemble. Enjoy using a piece of history.

Note:
1. The batteries are connected to the charging board directly via the solder tabs. You will need to cut the tabs and then desolder the leftover pieces from the board. The new batteries (with tabs) are unlikely going to have tabs long enough so you will need to solder some pieces of wire to the tabs and then connect them to the charging board.
2. Insulate all joints (wires to tabs) with heat shrink or some other insulating material.
3. Overall this is a very simple task (20 min) if you have the skills required.