The rock molds are HO scale, and located with the other scenery & layout products. Usually in the same isle as plastic models & paints, or the next one over. Your store may just be sold out.
Expensive items like the high end airbrushes/compressors & most other things over @$100-$150 aren't eligible for the discount. Strangely glues we'd most often use (mod podge or gallon sizes of elmer's white glue) aren't either.
Ground foam, bulk packs of trees, lichen, various wood for scratch building, etc etc are great at the discount price. It also pays to go down every single isle, my wife and I find something new all the time in the strangest places, that would be of use on a layout.
Also, don't forget to peruse the clearance isle! A few weeks ago I bought a Tamiya rotating paint stand, new in the box for $6...it's definitely nice, but I wouldn't have bought it at its full retail of $35-$40! The same day the clearance isle provided Model Master enamel weathering stain markers in "grime" and "concrete" for $.79 cents each. I think they're regularly around $7.00
Any time my wife is as eager to go shopping as I am, it's a good thing.
Tom