I'm not sure if an arts & craft store like Michael's or maybe the Hobby Lobby might have what you're seeking.

mostly a lurker, learning before starting a terriarium.
I'm not sure exactly what you are looking for, but it might be worth looking at dollhouse or train hobby sites. Most of this sort of thing for those scales is made of resin so waterproof and not too diffcult to drill through

I did a quick google search for terrarium waterfalls and just click shopping results. There are a lot and usually with some of the sights they are being sold by either professionals or the items owner and they could help with any questions.