I boight a 4S battery from an Amazon seller for £8. It works but clearly isn’t the same quality / longevity as the original. Paying more from a trusted supplier would be well worth it.
There is only one trusted supplier… Apple. Obviously you will have to take/send your phone to them though, they won’t just sell you a battery. No-one else will sell you a genuine Apple battery (if they claim to they’re lying).
Plenty of negative reviews of iFixit batteries if you Google. I would not be surprised if they were exactly the same batteries that eBay/Amazon sellers use, just more expensive. Their guides are excellent though!
FWIW I’ve always just used an eBay seller with high feedback & lots of sales of that battery. I realise that’s absolutely no guarantee of quality but I’ve never had a problem. YMMV. (Always use genuine chargers).