Where could i buy a cheap battery for my nikon d5100? 3rd party or authentic?

I want to get a battery grip but need another battery and can't find a battery that is 3rd party and will work with the D5100.

Stick with the Nikon batteries. The 3rd party batteries are often inferior.

If you read your user manual, you will see that generic batteries are not recommended for may reasons.

Since an original Nikon Li-ion battery costs only $41, it is a good investment

http://www.nikonusa.com/...ttery.html

I have two original Nikon Li-ion batteries used on my Nikon D100 that after all these years still work just like they did when new.

I can only speak for myself, but in my experience I have not had a problem with my 3rd party batteries