I use these disconnects from McMaster Carr for nearly every line in my CO2 system.
5923K43 - $6.08
5923K73 - $6.42
Good prices, but not great.
These disconnects are valved, so you can disconnect them without losing pressure. They have integrated poppets that shut flow off when they are disconnected. By the way, these are manufactured by Colder Products.
If you really want to save some money you can get non valved versions. The downside- no automatic shut off. The upside- crazy savings!
5923K41 - $1.73!
5923K71 - $1.27!
Also, the non-valved versions are interchangeable with the valved versions, so you can mix and match.