What battery to purchase?

Hey, I’ve got a ASUS ZenBook UX303U, and the laptop is dying pretty quickly now.

Assuming that I’ve had the laptop I assume it’s just the battery going bad. I found the post on there describing how to replace the battery for this laptop, but I couldn’t find anything stating which battery to buy, or where to get it from.

ASUS states that they do not sell a battery for this laptop. So which battery should I buy for this laptop?

Thanks!