Big Motors $10
I see a variety of the large "Power Wheels" motors on Ebay from... China. See https://www.ebay.com/itm/173852487435 and related items. Real Power Wheels motors are hard to find. Jon @xanadu used these on a big outdoor autonomous 4G GPS rover a while back. Food for thought if you need something big. Will take some work to couple a wheel to it.