You can still find things like an Arduino Micro that has less ROM & RAM than the lunar lander.

But finding something new slower than a 2MHz CPU is probably a challenge nowadays; even the Micro is 16MHz and can probably be overclocked a ways above that without much work or risk.

https://www.amazon.com/dp/B00AFY2S56 - not that that is necessarily the best way to buy such a system, just showing they exist.