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.