What are your tank's footprint dimensions? A 15lb. bag of Flourite will probably give you a 3 - 4" layer which is not all that deep. Or you could ask your LFS if he has a broken bag laying around and pick up maybe 10 - 12lbs. or so.
This clay based material is worlds better than most inert stuff you'll fine in LFSs.
You won't be sorry for laying out the extra dollars it will cost.
Len