This question get asked a lot of times, so let me try to clarify the situation.
When we started with the Spine, we made a battery based on a stock LIPO battery from Hobbyking. We quickly learned that the batteries did not really work well in a vehicle because of the harsh environment and conditions they are driving in.
So the next step we did was to improve the safety of the batteries, but it included a lot of work and was not really scalable and also our customers would probably not pay for us to do this work. And we could also not afford getting a custom battery built for us, so things were a little tricky.
The last step we did was to base our batteries on 18650 cells, and to make them fit nicely into our existing enclosure. Now some few users could make the battery themselves, but the average person does not have the equipment needed for doing a proper battery pack. On the other hand we had the equipment, but it takes us more than an hour to make the battery, so it would be too time consuming.
The hard truth in this industry that we are fighting against is that everyone makes their own custom batteries, electronics etc, and there is no good and safe choice for a project like ours. So this is why we stopped selling batteries, its too much work for us, and in case something would happen with the battery for a customer, it would be a big disaster for us.
So here is what we can recommend.
Use a company specializing in making custom battery packs like:
Do the work with 18650 Cells yourselves:
Use a non solder/welding solution
Ebikeschool also has another website with a solution, did not manage to find it right now.
Do like we used to do. Buy from hobbyking and wrap them in protective cover. https://hobbyking.com/en_us/turnigy-5000mah-6s-20c-lipo-pack-w-xt-90.html
Wait for us to make a proper commercial and approved battery. It will around the end of the year and will not be compatible with the current Spine format.