What you want to do is invest money in one of these battery cases
Then you can install and swap out 18650s to your hearts content.
Invest money in good panasonic, Samsung or Sanyo 18650 cells and they should keep you going for at least 2-3years before they are completely spent.
I bought my panasonic's 2 for £12 off amazon a while back but they've discontinued them and all the other stuff you find on there are unreliable china imported tat made for vapes and e-cigs which run the risk of exploding or starting a fire when or if they short out.
Only other place i can think of is
Ebay but you'll likely to come across counterfeit batteries. The only person I really trust is
Torchythebatteryboy -- He used to sell battery packs for lights and i have bought two of them in the past but it seems he's stopped selling them now. He sells samsung cells though they've been repackaged with his livery so theres a small chance that he could also be selling you fake batteries too... (ive known him to re-brand chinese lights and sell them for profit... but in his defence they were pretty decent lights)
Either way £10.99 for two samsung 3000mAh 18650 batteries is pretty good.
I would trust Torchy more than other battery sellers on ebay.
::EDIT::
Pete beat me to it 👅