Good CO detector?

HVAC adjacent question at least

The furnace in our house is pretty old. Something like 25 years. Works ok even if it’s not super efficient, but not in a place financially to get it replaced. I worry about the heat exchanger cracking and killing is all in our sleep

Any professional hvac folks in here that can recommend a good quality detector that starts alerting us at lower levels, before we are too poisoned to move?