I still haven't stumbled into a source (or means to deliver) large rocks without breaking the bank. I'm not looking too hard - just hoping that they'll fall (figuratively, not literally) into my lap.
My brother mentioned that they were throwing away a couple of large shipping crates at his work. I figured they'd make a good substitute for a couple of big rocks. I placed them on top of some scrap pressure treated stuff I had laying around to slow the rotting process and used some other scrap for a couple of ramps for variety. Should be good for some fun, and since it's all untreated wood, it'll make a nice bonfire when it starts to fall apart.
Coincidentally, my neighbor brought home a couple of loads of wood recently too. With a little nudging around, they turned into a nice "rock" pile.
I really enjoy riding at home. Lately, my daughter has been joining me on her XR50 which is a huge grin factor.