Apparently there was a reason I had such a difficult time finding Monster Cereals: they're on sale exclusively at Target this year. It's possible this has been the case in previous years and I simply didn't notice the trend, but dealiciousmom.com had this to say about this year's availability:
Available for a limited time, Count Chocula, Boo Berry and Franken Berry have added spoonfuls of Spooky Fun to breakfasts for more than 40 years. Make sure to grab your boxes of Monster Cereals before they disappear for another year because once the boxes are gone – they are gone for good! Plus at only $1.99 a box this is a nice little way to bring back memories and create some new Halloween traditions in your house!
Count Chocula, Boo Berry and Franken Berry will only be available from September 26-October 31 at your local Target checkout aisle.