I got a bunch from my backyard actually. you just have to know what kind of rocks you're putting into the tank. Pennsylvania is nuts with the amounts of limestone we have. you should only use inert rock in a tropical aquarium or something that wont alter the water parameters
Quartz, petrified wood, lava rock, late is ok to name a few. If you you touch the rock and it leaves a white powdery residue on your hands, its no good to uses. Limestone and other calcium based rocks will make your hardness skyrocket and your ph climb. Find a stream by you and use smooth river stones. They look natural and most are very safe. I'm sure someone will bash me for disrupting an ecosystem, but its a couple rocks....the ecosystem will be fine.
Wow sorry about the spelling. The t9 on my phone is a pain in the a$$. I meant slate is ok to use. I also wanted to add that I heard the chemical pH down by API works to check rocks. Put a few drops on the rock and if it fizzes its no good to use.