Buy batteries from Batteryhookup or Home Depot tool/equipment/mower/drill batteries which 5s or 21v fully charged for the small bricks, or higher voltage bricks for the big stuff, like EGO or is it ECO from Home Depot. Can find good sales at HD, Ryobi's go on sale but its a matter of how will you mount them and how will you charge them? People have3d printed mounts and bought the charger that goes with the battery.
Used to be able to buy used Makita tool batteries for a half decent price from Quebec es member.
Charger will be required, a charger with half decent charging amps at a safe rate.
Lead is dead! Your 12ah in lead is only really something like 8 or 9ah of useable power, plus they are heavy and awkward, where you going to mount those beasts!? Batteryhookup might be your best cheap bet, or find a deal on some stand up scooter rental batteries which arent high discharge as they are usually 36v and low power so maybe like 10 or 15amp for a Bird/Neuron scooter. Maybe the bicycle rental batteries are better, they look much larger but surely they are weak amp discharge.
For that setup, might be wise to do an external controller for kooling reasons. Motors get hot, controllers get hot, put both in a confined closed tin can and its just a lot of built up heat. 24v seems logical, then gear it with a gear on the wheel being big for more torque with a smaller drive gear on the motor. Like a 22t crank gear and a 42t rear gear for hill climbing, same thing. Torque!