I tried eBay like 4 times. But they were all fake or defective. At least one of them was dead on arrival.

I finally bought one from iFixit.com. Far more expensive than eBay, but the battery actually worked great for about a year. Then about a month after the 1-year warranty expired, it degraded noticeably (maybe to 80%) with only about 100-200 charge cycles. Even iFixIt cannot source a battery as good as the original Apple.

Right now, it's at 65% capacity after 584 charge cycles, after 4.5 years of service. I will probably go back to iFixIt. At least I'll get one year of full capacity from them, instead of the fake or DOA ones from eBay.