18650 and 26650 lipo cells are readily avail. Most old laptop packs are full of good 18650s and one dead one. I run them in monster flashlights. I just had to rebuild an Ingersoll Rand 19V lipo pack that is no longer avail
One dead 26650 cell took a $200 battery out of commission. Cost $20 to fix as I had to buy a 2 pack of the Sony cells. They were surprisingly easy to solder tabs on w/ the nickel plate (still need to be FAST w/ a big iron.)
For a non-round solution there are tons of RC airplane/car lipo packs but likely wont fit as well and have no protection. Some SCARY amounts of energy storage avail there. I have 100+Wh packs now
. It is what I was going to convert the IR pack to if push came to shove.
Edit: FYI, 18650 = 18mm OD x 65mm long. No idea why the trailing zero. 26650 = 26x65.