How 'bout "Gardening supplies" :
https://www.youtube.com/watch?v=sXb8rJ8Rm3I
I don't know how true his claim of "super non-toxic" is; I've read some claims to the contrary regarding diaper materials, but I suppose that's a discussion for elsewhere.
Alternatively, you could say that you are using the superabsorbent polymer crystals and cellulose fibers from the diapers to create an ersatz napalm. At least that was my ready excuse when I bought adult diapers at walgreens, along with a bottle of 91% rubbing alcohol, but the girl who rang me up didn't even show a hint of either curiosity or bemusement, So I decided to just not embarass myself with any nervous sounding excuses.
Of course, if your package contains adult size diapers with teddy bears and rainbows printed on them, and anyone is going to actually see them, well... just suck on your paci, and shake your rattle menacingly at their stupid face until they go away! Stormageddon fears no giant mortal!