Package Size 90x 76x 25 / 3.54x 2.99x 0.98inch, Charge Voltage 4.2V, Max. Discharge Current 3.5A, Nominal Capacity 3600mAh, Internal resistance (m&Omega) <80, Nominal Voltage 3.6V, Battery Size 18.6x 68.5 / 0.73x 2.70inch, Charging current by charger 1A (recommended) 2A (maximum), Charging current by on-board USB port 1A, Chemistry Li-ion, Minimum Capacity 3500mAh, Model 18650USB-3.7-3600, Weight 230g. Just check coupons out (with price alert and history). Let’s see what best price is ATM.
Aliexpress
I could not find it on Aliexpress. Click here to search by yourself.
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. One coupon is available. Cool!
Link: Banggood product page
1. ($21.99): BGtorch9
just apply this code in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): BGAlerabatI
(probably won’t work, but you can try them out)
Price history and price alert
Check out price history chart (for all/most countries) for Soshine 4pcs 3.7V 3600mAh 18650 Battery. The lowest price was: $21.99.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).