Carbon Monoxide alert devices are sold just about in every big box store including Lowe's and Home Depot. They have also recently been combined with smoke detection so you will have an all in one unit. Please know that these are not lifetime units... there effectiveness is generally around 5 - 6 yrs and they will need to be replaced.
Radon is another matter altogether. Radon is an invisible gas that comes from small amounts of uranium found in soil, sand, and rocks. It can even be found in your water if you have a well system (water that comes from surface water, like a reservoir, usually doesn't have a radon problem) -- you inhale it each time you take a shower. It seeps into your house through cracks and holes in the foundation and basement. People think that newer homes are radon-proof, but in fact they tend to concentrate the radon even more since they are so well insulated and sealed from outdoor air -- once the radon seeps in, it gets trapped there and can't get out very easily. Some States are of more concern for Radon than others, but every State has tested for Radon. If radon is in your house, your chance for lung cancer goes way up -- in fact, more than 20,000 people die each year from cancer caused by radon. The EPA estimates that 1 out of 15 houses in the US has high levels of radon.
There is no device that will alert you to Radon Gas, you can however purchase Radon Testing Kits which will require you to leave a test kit in your basement than seal it and send to a lab for analysis.(again found at Lowe's, Home Depot and Target) I checked the Internet and found Target has a test kit for about $20.00 which uses the activated charcoal method for measuring radon -- includes return mailer envelope with NO additional charge for lab analysis. Be sure to visit the official EPA site for additional information on radon dangers, radon testing, and radon abatement: EPA.gov/radon.
Not sure about any-type of radon detector but you can buy a good carbon monoxide detector at Lowes or Home depot..I bought the type that plugs into the way outlet for around 20 dollars..