u can get from river or waterfall but make sure u got them higher up stream.
Also remember to clean it because you dont know what kinda bacteria or germs it might contain.

This is what i did to my waterfall sands.

1. wash clean with water
2. Pour boiling water on it
3. spread it out on the floor and let the sun dry them

after this only i will put it in my terrarium but after the above process most of the nutrient will be gone.
u will have to start from zero.

i believe most LFS will carry those background.
Dont cut them into half, its much more hassle free if u remove the middle glass bracer and put the background in.
afterwards juz glue back the middle glass bracer with silicone.

however if ur doing riparium, having the middle glass bracer is kinda ugly in my opinion.

ps; remember to silicone the background properly to avoid it coming off after everything been setup.