Dude, just say, "No." Gravel is a bad idea for many reasons. Think about it: your roots will be growing in it. It's not sterile, and doesn't hold moisture as well as hydrocorn. I'm sure you could find a place to buy some online. If you're paranoid about buying hydroponics stuff online...idk, buy some tomato seeds or something to make it look less suspicious.
Why the hell would someone be afraid to buy hydro stuff online? Has anyone, anywhere, EVER been busted for buying hydro supplies online? NO!
Law Enforcement would never, ever get a judge to sign a search warrant based on some citizens hydroton medium order. You paranoid hippies are almost too much man!
Do what I did. Go to college for a couple years and get a degree in Law Enforcement. I'm not a cop and don't plan on being one, but isn't it convenient for me to know more about the law then most cops actually do? Having a degree in anything let alone law enforcement is very rare as a prerequisite to be hired as a police officer. Most cops have nothing more than a high school education and 16 weeks of police academy.
And regardless, anyone can buy anabolic steroids on about a dozen different websites and have them mailed right to your door. Do you really think LE is after you and your bag of hydroton???? Get real. Order some medium.......