Found at Office Works, also sometimes seen elsewhere, are mobile phone boost chargers or power banks which contain 18650 polymer lithium ion batteries.
Once the stock is old, they are often discounted. At the right price, they can be cheaper than buying them elsewhere. As an example, the other day I got a Comsol 6600 mAh Power Bank PB-01-06600, discounted to $20.
It contains three 18650 cells, in parallel, probably assembled from matched cells. A short cable ends in a JTX connector, the same used by Sparkfun and Adafruit, but reversed polarity. It also has a charge and protection circuit, indicators and pushbutton switch, but I didn’t need them.
I also didn’t need the case, but this can be removed with a rotary cutting tool; I used a Dremel with a generic cutting bit 7mm diameter, 1mm width, with 3.11mm shank. I cut around the edge, being very careful not to cut deep enough to hit the cells. (7mm minus 3.11mm divided by two gave a good clearance; it was difficult to dig too deep because the shank would strike the case.) Then the case can be broken open with screwdriver.
The cells are anchored to the case with double sided adhesive. The cells can be disconnected from the PCB, and then slowly removed.
Reversing the polarity of the JTX connector takes care, because without a pack protection circuit there’s a lot of energy that could be delivered if the contacts are shorted.
I used a piece of copper wire inserted from the rear of the connector to unlatch one of the contacts, drew it out, insulated it with tape, then unlatched the other contact, inserted it on the other side, removed the insulating tape, and then reinserted the first contact into the opposite side.
Now, they power a SparkFun ESP8266 Thing each. For many days.
Disclosure: I’m not connected with the above shops or brands except as a customer.