TOPIC: CO Test Equipment

Hope you all had a Great Thanksgiving
I am in the process of getting my HI Business set up in Northern Colorado.
I am checking out the CO Detector Test Equipment and I need to purchase one.
I do have the TIF 8800X Gas Detector but I did not see any CO Detector available from TIF.
For those of you who use one.....What are you guys using and what is a very good quality and affordable tester which can be relied on??
Thanks