You seem to have two questions overall:
- What can be the best option online to buy a most lasting and robust battery replacement?
- How can I ensure before ordering that it's closest to the genuine battery possible?
The first question doesn't work here. I can appreciate your predicament but understand that answering it well means it's: a) likely specific to where ever you live; b) only relevant for a short period of time because prices changes, retailers come and go, technology marches on.
The second question seems okay though -- when ordering a battery online, how can you ensure it's either genuine Apple or at least Apple compatible to some degree? That's a question with some general longevity to it, especially if you focus the question on how, when doing DIY repairs to Apple equipment, to ensure you're buying the right components and not ending up with partially-compatible pieces. It could even be made non-specific to batteries. That's a good question IMO.
I'd suggest shortening the whole thing down to something like:
How do I make sure I'm buying Apple-compatible replacement parts when performing DIY repairs?
I live in a place where Apple has no official presence. Support and repairs are done by Apple-authorized service providers. I'm fairly savvy with electronics and would like to save on the authorized repair markup and do my own repairs. I'm currently in need of a battery replacement in my MacBook Pro, but other repairs may come up as I try to extend the life of my Apple investment.
How can I be certain that any replacement parts I order for my repairs that are obviously Apple genuine parts, are at least Apple compatible parts?
That seems to capture the spirit of your question, not wanting to order the wrong thing when going DIY in a country where returning an order can be problematic and expensive, but without asking for specific purchase advice that would quickly become obsolete.
Does that help?