There are several sellers on Ebay who would sell you used transponders in new uncut keycases. They will tell you that the onus is upon the purchaser to ensure that the key type is identical batch number as your existing serial number key. They tell you that you will need to have the blank key cut using your existing key as a pattern and then go to a specialist who can reprogramme the chip so that you save money and some even tell you thaat you are saving the planet by re-using the old keys instead of polluting the atmosphere.
All of this information is useless and will cost you. The used key will be anything up to £25, copying the cutting £10-£20, new battery £5, toatl about £50. After much searching you may find one or two people who can recode the receiver in the car but they need to remove it from the car , clear and re-write the chip all for about £250. What have you got now? You've got a useless original key and a second hand re-programmed key which is approaching £300.
Contact your local dealer, quote the car's VIN ( vehicle identification number) and pay about £130 for a brand new key ex-factory...guaranteed to work.
These second hand remote key sellers should be banned from Ebay and certainly avoided at all costs.