Where can you buy a marble shower threshold?
I have googled and checked out my local big box store. I am stumped.
and how much do they cost?
You can use a door threshold if it is long enough, but any good tile supply store should have them. You might also check with a stone fabricator...you might be able to pick up a piece of scrap for next to nothing.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013