I don't know why you want "genuine Samsung" - Samsung doesn't make batteries, they buy them from battery manufacturers.
I've found the Anker batteries to be very good, usually fresh and they're new. One for an S5, with NFC, is $12.99. (If you'll never use NFC, a battery without the NFC antenna is $8.99. Order other stuff you need, to bring the order up to $30 or more (I place my "small stuff" Amazon orders when I need Litter Genie refills for my cat - that one item is over $30, so it's free shipping on all the little $1 and $2 pieces.)
Or you can get 2 NFC batteries and a charger (to charge both the one in the phone and the one out of the phone at the same time) for $25. Swap every month and you'll be throwing the phone away before you need another battery (as long as you don't discharge either one lower than 40% - carry the second one as a spare, so you can swap if the one in the phone gets to 40% and you still need your phone before you'll be able to recharge it.) My V551 original batteries still give me bout 95% capacity - they're from 2003 or 2004. Amazon has the battery listed by some vendors for more than the phone is worth. When your S5 is only worth $10, and the batteries are still working, you'll have an "emergency spare" phone. And if you lose it or it gets stolen or broken, you won't care much. (I can replace my V551 for about $20 from Craig's List if I don't mind getting ripped off.)