The two chassis I got direct from China are now available on Amazon. $15 gets your choice of beautiful aluminum chassis kit with two DC motors PLUS the coveted motor controller board which drives two motors using servo-style pulses. Essentially converts DC motors into CR servos. Mikey Likey. Mine are red, these are blue. I'll have to get blue ones too, I guess.
blue rectangular https://www.amazon.com/gp/product/B06XZC2XDV
uses gutted 9g servo-style gearmotors, works fine. Heavy-duty caster seems out of place but works.
blue round https://www.amazon.com/Feetech-FT-DC-006-SMC-Aluminum-Vehicle-FT-SMC-2CH/dp/B06XXS8L1C
uses Solarboticsstyle gearmotors in several different gear ratios, 48:1, 120:1 or 256:1 but the description doesn't say which. I favor the 120's myself.
There are a few Feetech offerings on Amazon as well, they look similar. There's a red round with no controller for $15 and a yellow rectangular with low-grade FS90R CR servos, I'd stay clear of those.
"When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."
- Pablo Picasso