I don’t know about a website who makes custom battery packs, but here in the forum some guys can make you a pack to your needs, just say make a thread with what you want and from where you are and i‘m sure you will get a pack done. A bms never bad. Just in case you didn’t know, in your vesc you can set a battery cut off limit too, so that you will not discharge your pack too much. Not only the bms can do so.
My commuter board uses (2) Multistar 6S battery packs in parallel. I’ve put about 60 cycles through them so far and have not seen any degradation or puffing. All commutes are tracked for mileage and battery consumption.
One thing to note is that I do balance the battery cells after every charge. Also placing the lipos at storage voltage when not using the board.