Where can you buy a marble shower threshold??

[Gramps]

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?

 

[Jadnashua]

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.