Going through Apple is the only way to get an OEM battery.
The eBay ones are original batteries that have been pulled apart and repacked, or even worse, 6 year old batteries that have had the onboard chips wiped so the machine thinks they're new, but they're actually used.
If you use programs such as Coconut Battery (or Apple's own system information tool), you know that there is no way to "wipe" the onboard chips. Even if you could, it would measure the capacity again within 1-2 charge cycles, and would report a low capacity in mAh (which is converted to a "Health" rating in macOS and in Coconut Battery). I've bought eBay batteries that are "genuine" but report manufacturers that Apple uses and have had no issues whatsoever. The people who use those computers have not complained once about the batteries. No swelling, good battery life, etc. Yea, they probably aren't quite as good as a genuine Apple battery, but they aren't the boogieman that so many people in the Apple community make them out to be.
Want to know my secret? I only buy batteries that have 100% PRODUCT feedback on eBay, from sellers with at least 98% feedback.
If you have lots of money and don't mind being without your Macbook for up to 2 weeks, take it to Apple. I take my Apple stuff for certain things and fix them myself for other things. It depends on what makes the most financial sense, warranty/AppleCare status, etc. But the mentality that Apple is the best place to take your Macbook is not always true, and is used as an excuse to justify Apple's anti-repair practices.