Pulled off the inside door panel this morning to see what was going on with the door not opening from the inside. The worse case scenario would have been that the cable from the inside handle to the latch would be broken. I did a little search on the Miata.net Forum and it seems like the cable is not available by itself, you have to buy the entire inside door handle assembly which included the handle and the two cables (one each for the latch and the lock.) The best case scenario would have been that the interior door handle itself had broken somehow.
I got lucky. Back a couple of years ago, just a couple months after we bought the CTBNL, I had worked out a deal where I got the OEM silver interior bits from a newer Miata to brighten up the cockpit some. I saved the old black pieces those items replaced, so I actually had an extra set of interior door handles in a box in the garage.
The top picture is of the back side of the passenger handle. The blue and yellow plastic pieces are where the cable ends hook into the mechanism. If you hover over the image you can see the arm that the yellow piece is on has physically separated the part you pull on to open the door.