Big Motors $10

erco

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.

xanadu

I'd love to hack one of these - https://www.bigtoysgreencountry.com/blade-xr-2-0-4x4-utv-ride-on-side-x-side-rc-w-rubber-tires-jet-black/

Not willing to spend the $500, but it's already RC which means bolting on solar, microcontroller and sensors and you're done.