The answer, surprisingly, is a packet of fags.
The cigarettes in question are Morleys. And you can't buy them in any shops although you may find the occasional packet turning up on ebay for prices in excess of $150. That's because Morley is a fictional brand used in film and TV to avoid problems with product placement or litigation. The packs you can buy are former props.
Morleys have a long and noble history. Here's a young William Shatner breaking open a packet in the classic 1963 The Twilight Zone episode Nightmare at 20,000 Feet.
And here's Darrell finding a box of Morleys in an episode of The Walking Dead.
That's at least 50 years of media appearances. The first I've been able to find is in the psychiatrist's scene at the end of Hitchcock's Psycho in 1960. They then pop up on TV in an 1961 episode of The Naked City. The following year they appear in an early episode of The Dick Van Dyke Show in January 1962 as chocolate cigarettes given to a child, bizarrely. From then on, they were everywhere.
So there you go. Arguably the most ubiquitous prop in TV and movie history is a packet of tabs. And Morleys aren't the only fake brand to have popped up across a range of shows. Quentin Tarantino films feature Red Apple cigarettes, Heisler beer has appeared in shows like Weeds, My Name is Earl and many others, Oceanic Airlines has appeared in shows like Lost, JAG, Fringe, Chuck and goes all the way back to an episode of dolphin-based TV show Flipper in the 1960s.
Oh, and if you like the whole 'surprising connections' thing, do consider getting hold of my first two books - Joined-Up Thinking and Constable Colgan's Connectoscope. Both contain chapters where every fact is connected to the one before it and the one after it. The facts also connect to facts in other chapters and even between the two books! Both books can be obtained from the usual online outlets (see links on the right of this page) and bookshops.