A 45" door is quite unusual, but then, an 8" marble threshold is a
bit unusual also.

There are many sources for thresholds, plain and fancy.

Here is one:
http://www.pemko.com/index.cfm?event=products.productListing&searchName =Lookup+by+Category&openFilter=loadCategorySearch& ratingIds=&categoryId=902&subcategoryId=&productMa terialId=

What is the door material?
Is the floor out of level?
Can the threshold be run with the floor and the door be custom cut
to work with the threshold?
How tall does the threshold need to be to solve the problem - will
it become a stumbling block?

Here are some solutions:
Set the threshold and cut the door to match
Get a threshold that matches the gap at its narrowest point and
shim the rest to door/floor grade. Well set thresholds are grout
packed anyway, so setting the threshold in wet mortar allows
custom setting the threshold. The edges can be caulked once the
grout has set.
There are automatic door bottoms that raise and lower. The amount
of travel is limited, but they are made to accommodate both uneven
gap and the need to clear flooring. When the door is closed, a
pin hits the jamb and pushes the weatherstrip down to seal to the
threshold.:


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DanG
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wrote in message
oups.com...
I need to install a threshold for a 45" wide door. The current
threshold is marble that is 8" wide and 45" long. It is
currently
flush with the floor and there is a small gap between the door
and the
threshold. I would like to replace this threshold with one that
is
raised from the floor to reduce that gap. Does anyone know
where I
can buy thresholds for this project?