Did your BMW dealer say that you needed the radio/navigation screen (the info display / on board monitor) or the Navigation computer?
In most cases the Navigation computer installed in the trunk fails and causes the on board monitor to blank out or light up and be grey with lines. This looks like the screen failed however most of the time it is the computer.
You should have no problem installing a used one. Just remember to take into account that the used one does not have any warranty and may soon fail as well. Regardless, this is the option I would take because you can buy several used computers for the price of a new one.
You can usually find the navigation computer or info display screen (depending on which part failed) on ebay at a reasonable price. Installaton of the computer is really easy and you should have no problem installing it yourself. Here is a good website with install instructions and a breakdown of the different options of navigation for E38, 7 series. Find out which model unit you have. There is Mk1, Mk2, Mk3, and Mk4.
On www.Ebay.com you can search for "BMW Navigation Computer" or "BMW Navigation". Just make sure you get the right one because most auction sites dont allow refunds or returns.
Or if you live in the USA do a search on http://www.craigslist.com/ and narrow down your state/city. maybe you will get lucky and find someone near-by that has one for sale.
If you dont want to do either of those options I would suggest calling around to your local junk yards to see if they have any junk E38 7 series.
Here is a website for USA that lists junkyards that have the part you need.
Just select your year/make/model and select "Radio/CD" from the drop down list. Then press search The next screen will let you narrow down which item you want to search for. Select the part you want and press search. You will then get a list of junkyards that have the part you need in your area.
I hope this helps