18650 cells have been commonly used for years in esk8 but most have moved onto 21700 cells now. Better energy denisty and power output for the amount of space they take up. That being said, there are several 18650s that are suitable for esk8. E.g. Samsung 30q, Murata vtv6 and,vtc5, Molicel p26a to name a few.
As was pointed out above, you do need to make sure that the power output from your battery is sufficient for your needs. I think before you consider building your own pack you need to do some reading… Like a lot of it. Here are a few threads you should start with. Building a battery is no joke and shouldn’t be taken lightly. It can be very hazardous and a risk to life and limb. Lithium fires are pretty much impossible to put out once they start.