Just for fun, (and because I have a calculator so it's not hard), thought I'd continue with the math. Using ThoughtElf's suggestion of dividing equally among the states to determine the average number of
outlets this time, 20,575 divided by 51, (the extra 1 is for Canada and skews the results to oppose rarity, imo), leaves us with an average of roughly 403 outlets per state and all of Canada. (Hat tip to Whiteangora for store count.)
So using the average 403 outlets in the entire state of Florida and the average of six rolls per store, that's around 2418 rolls of tape in the state. Now if the entire state lived in the Orlando area, that would end up with around .001 rolls per person. But that's not the population of the entire state.
The Census Bureau, here, estimates the population of the state as over 18 million people which would put it at around .0001 rolls per person. At best, the averages show that maybe 1 in 10,000 people would have a roll of this tape.
Again, I suggest that the bulk of those rolls went for more professional uses rather than the average homeowner which would increase the odds dramatically, imo.
Also, like ThoughtElf, I think there really is a lot less than 1 in 10,000 that would have also had access to the Pontiac and most importantly, access to Caylee. (Not to mention the other factors that would have to be in play.) In fact, I think adding these other considerations, that puts the odds at around 1 total in the world.