Im sorry I had to. But for serious go for a walk thru the woods, look around construction sites, edges of the yard, of course you could always find specialty rock in the internet, but I'd dig around thru the woods if you have and near by, or even hit up a landscape supply store
Visit a gravel pit. Talk to the guys. They discard rocks all the time. There's usually a pile of them. They might just let you take what you want....if you find something.
Definitely no lava rock around here! I have been looking around wherever I go, most rocks I see are so smooth though. I have found a few smaller ones that look like limestone. I will start to look near water more.
Instead of waiting to find the perfect rock, I prefer sticking stones together to form shapes that look interesting, leaving gaps at places where needed. For this, I use an epoxy sealant, meant for plumbing. It hardens in 30 mins. This way, I feel more in control and possibilities are endless. All the pieces shown below were assembled like this. Its not as good as porous volcanic rocks but something is better than nothing. Alternately, you could purchase such rocks online.