New Battery for One Plus One

Hi all, I've noticed that my battery, surprise surprise, isn't what it used to be (the phone is about 4 or 5 yrs old). I've installed acubattery which tells me that it is still 77% which is great but I'm considering buying a new battery to replace the old one. So, my questions are: -is it a good time now that is has degraded a bit (don't be fooled by that 77%, it discharges pretty quickly i have to charge twice a day even if I've done my best to optimize it) -i seem to understand that they don't produce this battery anymore, has anybody got a decent one from ebay or amazon or anywhere else (I'm based in the UK btw) Thanks