How can a person receive an expired life vest?
You can buy aircraft life vests pretty inexpensively for the purpose of having one or seeing how they work. 

There are a bunch of used ones for sale on eBay, they may have been taken from planes over the years or they may just find their way into circulation after being removed (before being trashed). 

Some of them get upcycled/recycled into other things so they presumably enter circulation at some point.