If you are looking for a mezuzah case, you can use Ebay or Froogle to find for a beautiful case for yourself.
However, if you are looking for the scroll to be fitted inside the case, it is not advisable to use anyone unless you know them to be reputable.
It is very easy for deceitful or ignorant sellers to sell you a scroll that is not kosher, because even small imperfections would make a scroll non-kosher.