As long as you find a battery for the iPhone 6 (and NOT the 6S or any other model) it should do the job. The 'standard' battery seems to be rated at 1810 mAh but longer-life versions, of around 2300mAh, are also offered for sale.
So simply searching eBay (or similar) for 'battery iphone 6' (and ignoring those for the 6s, which the search will also find) should be all that you need to do.
Prices seem to range from about £6 up to around £20. As there's no guarantee that paying more will get you a better product, my own instinct in your situation would be to look for a cheap battery from a seller with generally positive feedback. e.g.
https://www.ebay.co.uk/itm/225192474213