Yes. Perhaps you noticed on your Internet searches that Il Duomo is also known as Santa Maria del Fiore which you should also use an alternative search term. Often the Web is not a great source for plans and can be frustrating. So...
A good place to begin is the Library's Architecture Research Guide (under "Research by Subject" on the Library home page). Use the tab at the top of this Guide called "Drawing & Plans" for specific suggestions. Be sure to add the term "plans" to your searches. Often books provide high resolution plans and elevations, so you may want to start your search in the Library catalog.
Also, listed on the Architecture Research Guide Drawings & Plans tab is the Avery Index where you can search for articles from architecture periodicals. Once at Avery, you can go to the "Advanced Search" feature and limit by choosing "plans" and/or "sections" under "Physical Description" to find only articles with plans and/or sections. Some of these may be in paper format only and you will have to request Library staff to get the periodical for you. Be sure to get all of the information about the article -citation information - to allow us to find it. Citation information includes the following: name of the periodical, the title of the article, the author's name, the year and month it came out, and any volume number information.
If you still are not finding what you need, be sure to ask a librarian to assist you.