260+ decent quality 18650 is going to cost a fortune. I strongly suggest you have a look for a local “off grid” power retailer, or someone catering to ebike and EV builders.
Here where I am in Perth theres a few shops, but alt-tech is one I have visited, had great help from the staff there when I discussed my plans with them.

I can get 4x 100ah Sinopoly LiFePo4 cells, an 80ABMS and a box to assemble them all in, for around $600 Australian.

If you still want cylindrical cells, have a look for “headway” cells. You can get nice holders to clip them together, and they bolt together with busbars - much easier than trying to solder or spotweld them. Available in 8ah, 10ah and 15ah varieties.

Julian, OH8STN.org Has some great videos on building lithium powerpacks. His videos are aimed at amateur radio operators, and working in the field but are very relevant for what you want to do