CR2032 batteries. Can you get rechargeables? What charger?
http://www.ebay.co.uk/itm/5X-CR2032-3V-Brand-New-Lithium-Button-Cell-Coin-Battery-Batteries-DL2032-/321000210733?pt=UK_ConsumerElectronics_Batteries_SM&hash=item4abd1a812dhttp://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=[CACHEBUSTER]http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=8895790http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=9210581http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?ff3=2&pub=5574889051&toolid=10001&campid=5336525415&item=321000210733&mpt=356607
with minimal planning, 20p each
http://www.ebay.co.uk/itm/10-20-50-100-pcs377-SR626SW-LR626-AG4-2032-CR2032-DL2032-LITHIUM-WATCH-BATTERY-/310419778132?pt=UK_Jewellery_Watches_WatchAccessories_SpareParts_SM&var=610087625476&hash=item484675ea54http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=[CACHEBUSTER]http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=8895790http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=9210581http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?ff3=2&pub=5574889051&toolid=10001&campid=5336525415&item=310419778132&mpt=356607
with patience and a box to keep spares in, less than 9p each
http://www.megabatteries.com/item_details2.asp?id=14509&cat_id=469&uid=1718
or $33 (+postage?) for rechargeables...
@MrE started
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http://www.ebay.co.uk/itm/5X-CR2032-3V-Brand-New-Lithium-Button-Cell-Coin-Battery-Batteries-DL2032-/321000210733?pt=UK_ConsumerElectronics_Batteries_SM&hash=item4abd1a812dhttp://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=[CACHEBUSTER]http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=8895790http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=9210581http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?ff3=2&pub=5574889051&toolid=10001&campid=5336525415&item=321000210733&mpt=356607
with minimal planning, 20p each
http://www.ebay.co.uk/itm/10-20-50-100-pcs377-SR626SW-LR626-AG4-2032-CR2032-DL2032-LITHIUM-WATCH-BATTERY-/310419778132?pt=UK_Jewellery_Watches_WatchAccessories_SpareParts_SM&var=610087625476&hash=item484675ea54http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=[CACHEBUSTER]http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=8895790http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?pub=5574889051&toolid=10001&campid=5336525415&mpt=9210581http://rover.ebay.com/roverimp/1/710-53481-19255-0/1?ff3=2&pub=5574889051&toolid=10001&campid=5336525415&item=310419778132&mpt=356607
with patience and a box to keep spares in, less than 9p each
http://www.megabatteries.com/item_details2.asp?id=14509&cat_id=469&uid=1718
or $33 (+postage?) for rechargeables...