Places where I buy batteries:

AA, AAA Alkaline Batteries

 

9V Alkaline Batteries

$1.19/ea for 8

https://www.amazon.com/AmazonBasics-Everyday-Alkaline-Batteries-8-Pack/dp/B00MH4QM1S/

 

 

12V Auto Batteries

For basic low cost replacements: Walmart or Home Depot

Ni-MH Low Discharge Rechargeable Batteries

Amazon Basics or Sanyo Eneloop from Amazon

18650 Batteries

AliExpress Liitokala Batteries (best price for value/will take a month for delivery)

https://www.orbtronic.com/ (one of the best performance batteries)

https://www.illumn.com/ (trusted site for li-ion cells)

Walmart EBL 18650 Batteries (low cost option within the States)