You can take a battery pack from one of those kickbike scooters, they're usually 40v.

Another option is ordering a battery pack, lifepo packs come in loads of sizes with built in bms and everything pretty cheap.

Batteries are definitely cheaper now.

But also, you can scavenge a lot of the parts from stuff like broken electric scooters. They go for practically nothing on Facebook marketplace.

I'm considering buying a couple and putting the parts from them into a power wheels toy car. For the lulz.