Good Pukani is really light, 5lbs might be too much - but some of heavier stuff is almost like driveway gravel, 10lbs might not be enough. I'd just buy rock you like off a hobbyist and go by what looks good - water changes are your filtration in a tank that small.
Tell anyone at Lowes or Home Depot it is for a fish tank, they will say they don't have it and send you to a pet store. A pet store will say they don't have it and send you to a hardware store. Pet stores at least know what they sell, you can probably find it in plumbing at the hardware store.