Somewhat to my surprise, there are, in fact, biological metabolic pathways that produce carbon monoxide. An enzyme that does this job is carbon monoxide dehydrogenase which pops up in various colours and flavours of bacteria and archaea. Usually it is used to oxidise CO into CO2 but it turns out you can run the process in reverse, given a suitable source of energy.
There are two problems. Firstly, some methanogenic microorganisms consume carbon monoxide, reducing it to methane using hydrogen. Any environment that naturally produces copious amounts of carbon monoxide seems likely to be colonised by such microorganisms who may well eat all, or most of it. On Earth, most such organisms seem to live around volcanic vents which are a source of carbon monoxide, but the biochemical toolbox they make use of could be used in other less hostile environments, too.
Secondly, it isn't immediately obvious what the benefit would be to any organism that expended energy to split CO2, but then didn't do anything useful with it. I'd expect it to make some use of the carbon, like turn it into carbohydrates (or whatever) but instead it would just have to let it waft away. That's a peculiar thing to do with a useful and expensive to produce material. Sure, you get oxygen out of it, but mostly what oxygen is useful for in biochemical pathways is oxidising stuff with carbons in it.
That sort of suggests to me that the only real practical source would be volcanic, but I don't know enough about volcanism to make any further suggestions in that direction.