First thing i would do. Is locate a park or somewhere that there is a stream. Then, take your significant other/best friend/family on a lovely day at the park. Then you can observe an environment similar to what you wish to recreate. There you can find your stone, with a lovely weathered look.
Depending on how much work you want to put into this, you could do a false background/corner. It will take a bit more time, but it's amazing what you can do. Check out the DIY section. It'll really get your wheels spinning.
I am Rowdaddy.
20L Convict "Bedroom"
1.5gal in progress
55 gal Paludarium/Vivarium coming soon