Baby Oxygen Monitors

After 3 months of having my son sleep in my arms, I have finally found the trick to getting him to sleep 2-3 hours alone in his crib for naps. This is the longest he has ever slept on his own. Unfortunately, he has to be on his stomach. I know he's capable of moving his head if his airway gets blocked because he's done it on my chest many times. That being said, what monitors would you recommend? Any tips? I'm thinking about investing in one to ease my mind. I constantly stare at the monitor and want to go place a hand on him because I'm nervous.