I would avoid batteries that are supposed to be OEM Samsung S5 replacements. Given the age of the phone and the fact that it and its accessories are long out of production, any genuine batteries will be old stock that has been sitting around for a long time. As such they are likely to have less than their original capacity especially if they have been on a shelf in a discharged state.
It's also very likely that many of the batteries being sold as "OEM" are actually fakes even if they come from known sellers on Amazon or Ebay.
I would second the recommendation of the Anker battery linked to by ironass above, I currently have one of those in my S5 and it is doing very well after almost a year. Still at full capacity and no swelling. The only caveat with this battery is it doesn't have NFC (I don't care but you might) and for unknown reasons it is no longer available in the US so if you're here you can't get one.
If the Anker isn't an option there are other third-party batteries available, but you need to do some research to make sure the product and manufacturer are reputable and have good reviews. Some of these will also not have NFC, but if that's not a deal-breaker just make sure they are rated for the same capacity as the original (2800mah) and are new/recently manufactured.
