I got my kits and carts from The Widow as well, I also like them, so I asked her the same question... she said that they are Joye's version of tobacco flavor (and the boxes of carts say TOB), so I'm curious as to why yours are labeled Marlboro. But if they are, The Widow sells Marlboro flavored liquid on her site, and said that the 10ml and 30ml bottles she sells come from Joye, so you might try a 10ml as a sample if she has some in stock to see if it's a match.
She also mentioned that Janty gets their juice from Joye, so they might be a good place to try.
I've found that Jane's Kick Juice Blend #5 seems almost exactly like the flavor in the original carts that I have, pretty good tobacco flavor with just a hint of cinnamon. Here's a link to the product page for the 30ml bottles:
electronic cigarette e-cigarette e-cig electric cigarette e-liquid : Special Blend #5 30ml
EDIT: Forgot to add, I don't think everyone's Marlboro is the same, but with a few exceptions most juice is probably made by a handful of manufacturers. I know Johnson's Creek makes their own, for example, so theirs would definitely not be exactly the same.
Also, you might just shoot The Widow a note via the contact link on her web site to ask if she carries the same juice, and if so in what bottle sizes...
Tim