Replacement battery iPhone 4s

Can anyone advise me where Can I get a GENUINE Apple replacement battery for an iPhone 4s?

I am and electronics engineer and well qualified to fit my own battery. I am reluctant to use a 3rd party battery or let Apple charge me £70 to do a 5 min job. The Apple store in Sheffield say take it to a retailer and let them do it (£70), but they won't sell me the battery. The Phone is out of warranty.

Thanks

Dennis